Merge commit 'smcv/beautify'

[ikiwiki] / doc / plugins / write.mdwn

Ikiwiki's plugin interface allows all kinds of useful [[plugins]] to be
written to extend ikiwiki in many ways. Despite the length of this page,
it's not really hard. This page is a complete reference to everything a
plugin might want to do. There is also a quick [[tutorial]].

[[!toc levels=2]]

## Types of plugins

Most ikiwiki [[plugins]] are written in perl, like ikiwiki. This gives the
plugin full access to ikiwiki's internals, and is the most efficient.
However, plugins can actually be written in any language that supports XML
RPC. These are called [[external]] plugins.

A plugin written in perl is a perl module, in the `IkiWiki::Plugin`
namespace. The name of the plugin is typically in lowercase, such as
`IkiWiki::Plugin::inline`. Ikiwiki includes a `IkiWiki::Plugin::skeleton`
that can be fleshed out to make a useful plugin.
`IkiWiki::Plugin::pagecount` is another simple example. All perl plugins
should `use IkiWiki` to import the ikiwiki plugin interface. It's a good
idea to include the version number of the plugin interface that your plugin
expects: `use IkiWiki 2.00`.

An external plugin is an executable program. It can be written in any
language. Its interface to ikiwiki is via XML RPC, which it reads from
ikiwiki on its standard input, and writes to ikiwiki on its standard
output. For more details on writing external plugins, see [[external]].

Despite these two types of plugins having such different interfaces,
they're the same as far as how they hook into ikiwiki. This document will
explain how to write both sorts of plugins, albeit with an emphasis on perl
plugins.

## Considerations

One thing to keep in mind when writing a plugin is that ikiwiki is a wiki
*compiler*. So plugins influence pages when they are built, not when they
are loaded. A plugin that inserts the current time into a page, for
example, will insert the build time. Also, as a compiler, ikiwiki avoids
rebuilding pages unless they have changed, so a plugin that prints some
random or changing thing on a page will generate a static page that won't
change until ikiwiki rebuilds the page for some other reason, like the page
being edited.

## Registering plugins

Plugins should, when imported, call `hook()` to hook into ikiwiki's
processing. The function uses named parameters, and use varies depending on
the type of hook being registered -- see below. A plugin can call
the function more than once to register multiple hooks.

All calls to `hook()` should be passed a "type" parameter, which gives the
type of hook, a "id" parameter, which should be a unique string for this
plugin, and a "call" parameter, which tells what function to call for the
hook.

An optional "last" parameter, if set to a true value, makes the hook run
after all other hooks of its type. Useful if the hook depends on some other
hook being run first.

## Types of hooks

In roughly the order they are called.

### getopt

        hook(type => "getopt", id => "foo", call => \&getopt);

This allows for plugins to perform their own processing of command-line
options and so add options to the ikiwiki command line. It's called during
command line processing, with @ARGV full of any options that ikiwiki was
not able to process on its own. The function should process any options it
can, removing them from @ARGV, and probably recording the configuration
settings in %config. It should take care not to abort if it sees
an option it cannot process, and should just skip over those options and
leave them in @ARGV.

### checkconfig

        hook(type => "checkconfig", id => "foo", call => \&checkconfig);

This is useful if the plugin needs to check for or modify ikiwiki's
configuration. It's called early in the startup process. The
function is passed no values. It's ok for the function to call
`error()` if something isn't configured right.

### refresh

        hook(type => "refresh", id => "foo", call => \&refresh);

This hook is called just before ikiwiki scans the wiki for changed files.
It's useful for plugins that need to create or modify a source page. The
function is passed no values.

### needsbuild

        hook(type => "needsbuild", id => "foo", call => \&needsbuild);

This allows a plugin to manipulate the list of files that need to be
built when the wiki is refreshed. The function is passed a reference to an
array of pages that will be rebuilt, and can modify the array, either
adding or removing files from it.

### scan

        hook(type => "scan", id => "foo", call => \&scan);

This hook is called early in the process of building the wiki, and is used
as a first pass scan of the page, to collect metadata about the page. It's
mostly used to scan the page for WikiLinks, and add them to `%links`.
Present in IkiWiki 2.40 and later.

The function is passed named parameters "page" and "content". Its return
value is ignored.

### filter

        hook(type => "filter", id => "foo", call => \&filter);

Runs on the raw source of a page, before anything else touches it, and can
make arbitrary changes. The function is passed named parameters "page",
"destpage", and "content". It should return the filtered content.

### preprocess

Adding a preprocessor [[ikiwiki/directive]] is probably the most common use
of a plugin.

        hook(type => "preprocess", id => "foo", call => \&preprocess);

Replace "foo" with the command name that will be used for the preprocessor
directive.

Each time the directive is processed, the referenced function (`preprocess`
in the example above) is called. Whatever the function returns goes onto
the page in place of the directive. Or, if the function aborts using
`error()`, the directive will be replaced with the error message.

The function is passed named parameters. First come the parameters set
in the preprocessor directive. These are passed in the same order as
they're in the directive, and if the preprocessor directive contains a bare
parameter (example: `\[[!foo param]]`), that parameter will be passed with
an empty value.

After the parameters from the preprocessor directive some additional ones
are passed: A "page" parameter gives the name of the page that embedded the
preprocessor directive, while a "destpage" parameter gives the name of the
page the content is going to (different for inlined pages), and a "preview"
parameter is set to a true value if the page is being previewed.

If `hook` is passed an optional "scan" parameter, set to a true value, this
makes the hook be called during the preliminary scan that ikiwiki makes of
updated pages, before begining to render pages. This should be done if the
hook modifies data in `%links`. Note that doing so will make the hook be
run twice per page build, so avoid doing it for expensive hooks. (As an
optimisation, if your preprocessor hook is called in a void context, you
can assume it's being run in scan mode, and avoid doing expensive things at
that point.)

Note that if the [[htmlscrubber]] is enabled, html in
preprocessor [[ikiwiki/directive]] output is sanitised, which may limit what
your plugin can do. Also, the rest of the page content is not in html
format at preprocessor time. Text output by a preprocessor directive will
be linkified and passed through markdown (or whatever engine is used to
htmlize the page) along with the rest of the page.

### linkify

        hook(type => "linkify", id => "foo", call => \&linkify);

This hook is called to convert [[WikiLinks|WikiLink]] on the page into html
links. The function is passed named parameters "page", "destpage", and
"content". It should return the linkified content.  Present in IkiWiki 2.40
and later.

Plugins that implement linkify must also implement a scan hook, that scans
for the links on the page and adds them to `%links`.

### htmlize

        hook(type => "htmlize", id => "ext", call => \&htmlize);

Runs on the source of a page and turns it into html. The id parameter
specifies the filename extension that a file must have to be htmlized using
this plugin. This is how you can add support for new and exciting markup
languages to ikiwiki.

The function is passed named parameters: "page" and "content" and should
return the htmlized content.

If `hook` is passed an optional "keepextension" parameter, set to a true
value, then this extension will not be stripped from the source filename when
generating the page.

### pagetemplate

        hook(type => "pagetemplate", id => "foo", call => \&pagetemplate);


[[Templates|wikitemplates]] are filled out for many different things in
ikiwiki, like generating a page, or part of a blog page, or an rss feed, or
a cgi. This hook allows modifying the variables available on those
templates. The function is passed named parameters. The "page" and
"destpage" parameters are the same as for a preprocess hook. The "template"
parameter is a [[!cpan HTML::Template]] object that is the template that
will be used to generate the page. The function can manipulate that
template object.

The most common thing to do is probably to call `$template->param()` to add
a new custom parameter to the template.

### templatefile

        hook(type => "templatefile", id => "foo", call => \&templatefile);

This hook allows plugins to change the [[template|wikitemplates]] that is
used for a page in the wiki. The hook is passed a "page" parameter, and
should return the name of the template file to use, or undef if it doesn't
want to change the default ("page.tmpl"). Template files are looked for in
/usr/share/ikiwiki/templates by default.

### sanitize

        hook(type => "sanitize", id => "foo", call => \&sanitize);

Use this to implement html sanitization or anything else that needs to
modify the body of a page after it has been fully converted to html.

The function is passed named parameters: "page", "destpage", and "content",
and should return the sanitized content.

### postscan

        hook(type => "postscan", id => "foo", call => \&postscan);

This hook is called once the full page body is available (but before the
format hook). The most common use is to update search indexes. Added in
ikiwiki 2.54.

The function is passed named parameters "page" and "content". Its return
value is ignored.

### format

        hook(type => "format", id => "foo", call => \&format);

The difference between format and sanitize is that sanitize only acts on
the page body, while format can modify the entire html page including the
header and footer inserted by ikiwiki, the html document type, etc. (It
should not rely on always being passed the entire page, as it won't be
when the page is being previewed.)

The function is passed named parameters: "page" and "content", and 
should return the formatted content.

### delete

        hook(type => "delete", id => "foo", call => \&delete);

Each time a page or pages is removed from the wiki, the referenced function
is called, and passed the names of the source files that were removed.

### change

        hook(type => "change", id => "foo", call => \&render);

Each time ikiwiki renders a change or addition (but not deletion) to the
wiki, the referenced function is called, and passed the names of the
source files that were rendered.

### cgi

        hook(type => "cgi", id => "foo", call => \&cgi);

Use this to hook into ikiwiki's cgi script. Each registered cgi hook is
called in turn, and passed a CGI object. The hook should examine the
parameters, and if it will handle this CGI request, output a page
(including the http headers) and terminate the program.

Note that cgi hooks are called as early as possible, before any ikiwiki
state is loaded, and with no session information.

### auth

        hook(type => "auth", id => "foo", call => \&auth);

This hook can be used to implement an authentication method. When a user
needs to be authenticated, each registered auth hook is called in turn, and
passed a CGI object and a session object. 

If the hook is able to authenticate the user, it should set the session
object's "name" parameter to the authenticated user's name. Note that
if the name is set to the name of a user who is not registered,
a basic registration of the user will be automatically performed.

### sessioncgi

        hook(type => "sessioncgi", id => "foo", call => \&sessioncgi);

Unlike the cgi hook, which is run as soon as possible, the sessioncgi hook
is only run once a session object is available. It is passed both a CGI
object and a session object. To check if the user is in fact signed in, you
can check if the session object has a "name" parameter set.

### canedit

        hook(type => "canedit", id => "foo", call => \&pagelocked);

This hook can be used to implement arbitrary access methods to control when
a page can be edited using the web interface (commits from revision control
bypass it). When a page is edited, each registered canedit hook is called
in turn, and passed the page name, a CGI object, and a session object.

If the hook has no opinion about whether the edit can proceed, return
`undef`, and the next plugin will be asked to decide. If edit can proceed,
the hook should return "". If the edit is not allowed by this hook, the
hook should return an error message for the user to see, or a function 
that can be run to log the user in or perform other action necessary for
them to be able to edit the page.

This hook should avoid directly redirecting the user to a signin page,
since it's sometimes used to test to see which pages in a set of pages a
user can edit.

### editcontent

        hook(type => "editcontent", id => "foo", call => \&editcontent);

This hook is called when a page is saved (or previewed) using the web
interface. It is passed named parameters: `content`, `page`, `cgi`, and
`session`. These are, respectively, the new page content as entered by the
user, the page name, a `CGI` object, and the user's `CGI::Session`. 

It can modify the content as desired, and should return the content.

### formbuilder

        hook(type => "formbuilder_setup", id => "foo", call => \&formbuilder_setup);
        hook(type => "formbuilder", id => "foo", call => \&formbuilder);

These hooks allow tapping into the parts of ikiwiki that use [[!cpan
CGI::FormBuilder]] to generate web forms. These hooks are passed named
parameters: `cgi`, `session`, `form`, and `buttons`. These are, respectively,
the `CGI` object, the user's `CGI::Session`, a `CGI::FormBuilder`, and a
reference to an array of names of buttons to go on the form.

Each time a form is set up, the `formbuilder_setup` hook is called.
Typically the `formbuilder_setup` hook will check the form's title, and if
it's a form that it needs to modify, will call various methods to
add/remove/change fields, tweak the validation code for the fields, etc. It
will not validate or display the form.

Just before a form is displayed to the user, the `formbuilder` hook is
called. It can be used to validate the form, but should not display it.

### savestate

        hook(type => "savestate", id => "foo", call => \&savestate);

This hook is called whenever ikiwiki normally saves its state, just before
the state is saved. The function can save other state, modify values before
they're saved, etc.

### renamepage

        hook(type => "renamepage", id => "foo", call => \&renamepage);

This hook is called by the [[plugins/rename]] plugin when it renames
something. The hook is passed named parameters: `page`, `oldpage`,
`newpage`, and `content`, and should try to modify the content to reflect
the name change. For example, by converting links to point to the new page.

### getsetup

        hook(type => "getsetup", id => "foo", call => \&getsetup);

This hooks is not called during normal operation, but only when setting up 
the wiki, or generating a setup file. Plugins can use this hook to add
configuration options.

The hook is passed no parameters. It returns data about the configuration
options added by the plugin. It can also check if the plugin is usable, and
die if not, which will cause the plugin to not be offered in the configuration
interface.

The data returned is a list of `%config` options, followed by a hash
describing the option. There can also be an item named "plugin", which
describes the plugin as a whole. For example:

                return
                        option_foo => {
                                type => "boolean",
                                description => "enable foo?",
                                advanced => 1,
                                safe => 1,
                                rebuild => 1,
                        },
                        option_bar => {
                                type => "string",
                                example => "hello",
                                description => "option bar",
                                safe => 1,
                                rebuild => 0,
                        },
                        plugin => {
                                description => "description of this plugin",
                                safe => 1,
                                rebuild => 1,
                        },

* `type` can be "boolean", "string", "integer", "pagespec",
  or "internal" (used for values that are not user-visible). The type is
  the type of the leaf values;  the `%config` option may be an array or
  hash of these.
* `example` can be set to an example value.
* `description` is a short description of the option.
* `link` is a link to further information about the option. This can either
  be a wikilink, or an url.
* `advanced` can be set to true if the option is more suitable for advanced
  users.
* `safe` should be false if the option should not be displayed in unsafe
  configuration methods, such as the web interface. Anything that specifies
  a command to run, a path on disk, or a regexp should be marked as unsafe.
  If a plugin is marked as unsafe, that prevents it from being
  enabled/disabled.
* `rebuild` should be true if changing the option (or enabling/disabling
  the plugin) will require a wiki rebuild, false if no rebuild is needed,
  and undef if a rebuild could be needed in some circumstances, but is not
  strictly required.

## Plugin interface

To import the ikiwiki plugin interface:

        use IkiWiki '2.00';

This will import several variables and functions into your plugin's
namespace. These variables and functions are the ones most plugins need,
and a special effort will be made to avoid changing them in incompatible
ways, and to document any changes that have to be made in the future.

Note that IkiWiki also provides other variables and functions that are not
exported by default. No guarantee is made about these in the future, so if
it's not exported, the wise choice is to not use it.

### %config

A plugin can access the wiki's configuration via the `%config`
hash. The best way to understand the contents of the hash is to look at
your ikiwiki setup file, which sets the hash content to configure the wiki.

### %pagestate

The `%pagestate` hash can be used by plugins to save state that they will need
next time ikiwiki is run. The hash holds per-page state, so to set a value,
use `$pagestate{$page}{$id}{$key}=$value`, and to retrieve the value,
use `$pagestate{$page}{$id}{$key}`.

The `$value` can be anything that perl's Storable module is capable of
serializing. `$key` can be any string you like, but `$id` must be the same
as the "id" parameter passed to `hook()` when registering the plugin. This
is so ikiwiki can know when to delete pagestate for plugins that are no
longer used.

When pages are deleted, ikiwiki automatically deletes their pagestate too.

Note that page state does not persist across wiki rebuilds, only across
wiki updates.

### Other variables

If your plugin needs to access data about other pages in the wiki. It can
use the following hashes, using a page name as the key:

* `%links` lists the names of each page that a page links to, in an array
  reference.
* `%destsources` contains the name of the source file used to create each
  destination file.
* `%pagesources` contains the name of the source file for each page.

Also, the %IkiWiki::version variable contains the version number for the
ikiwiki program.

### Library functions

#### `hook(@)`

Hook into ikiwiki's processing. See the discussion of hooks above.

Note that in addition to the named parameters described above, a parameter
named `no_override` is supported, If it's set to a true value, then this hook
will not override any existing hook with the same id. This is useful if
the id can be controled by the user.

#### `debug($)`

Logs a debugging message. These are supressed unless verbose mode is turned
on.

#### `error($;$)`

Aborts with an error message. If the second parameter is passed, it is a
function that is called after the error message is printed, to do any final
cleanup.

If called inside a preprocess hook, error() does not abort the entire
wiki build, but instead replaces the preprocessor [[ikiwiki/directive]] with
a version containing the error message.

In other hooks, error() is a fatal error, so use with care. Try to avoid
dying on bad input when building a page, as that will halt
the entire wiki build and make the wiki unusable.

#### `template($;@)`

Creates and returns a [[!cpan HTML::Template]] object. The first parameter
is the name of the file in the template directory. The optional remaining
parameters are passed to `HTML::Template->new`.

#### `htmlpage($)`

Passed a page name, returns the base name that will be used for a the html
page created from it. (Ie, it appends ".html".)

Use this when constructing the filename of a html file. Use `urlto` when
generating a link to a page.

#### `add_depends($$)`

Makes the specified page depend on the specified [[ikiwiki/PageSpec]].

#### `pagespec_match($$;@)`

Passed a page name, and [[ikiwiki/PageSpec]], returns true if the
[[ikiwiki/PageSpec]] matches the page.

Additional named parameters can be passed, to further limit the match.
The most often used is "location", which specifies the location the
PageSpec should match against. If not passed, relative PageSpecs will match
relative to the top of the wiki.

#### `bestlink($$)`

Given a page and the text of a link on the page, determine which
existing page that link best points to. Prefers pages under a
subdirectory with the same name as the source page, failing that
goes down the directory tree to the base looking for matching
pages, as described in [[ikiwiki/SubPage/LinkingRules]].

#### `htmllink($$$;@)`

Many plugins need to generate html links and add them to a page. This is
done by using the `htmllink` function. The usual way to call
`htmlllink` is:

        htmllink($page, $page, $link)

Why is `$page` repeated? Because if a page is inlined inside another, and a
link is placed on it, the right way to make that link is actually:

        htmllink($page, $destpage, $link)

Here `$destpage` is the inlining page. A `destpage` parameter is passed to
some of the hook functions above; the ones that are not passed it are not used
during inlining and don't need to worry about this issue.

After the three required parameters, named parameters can be used to
control some options. These are:

* noimageinline - set to true to avoid turning links into inline html images
* forcesubpage  - set to force a link to a subpage
* linktext - set to force the link text to something
* anchor - set to make the link include an anchor
* rel - set to add a rel attribute to the link
* class - set to add a css class to the link

#### `readfile($;$)`

Given a filename, reads and returns the entire file.

The optional second parameter, if set to a true value, makes the file be read
in binary mode.

A failure to read the file will result in it dying with an error.

#### `writefile($$$;$$)`

Given a filename, a directory to put it in, and the file's content,
writes a file. 

The optional fourth parameter, if set to a true value, makes the file be
written in binary mode.

The optional fifth parameter can be used to pass a function reference that
will be called to handle writing to the file. The function will be called
and passed a file descriptor it should write to, and an error recovery
function it should call if the writing fails. (You will not normally need to
use this interface.)

A failure to write the file will result in it dying with an error.

If the destination directory doesn't exist, it will first be created.

#### `will_render($$)`

Given a page name and a destination file name (not including the base
destination directory), register that the page will result in that file
being rendered. 

It's important to call this before writing to any file in the destination
directory, and it's important to call it consistently every time, even if
the file isn't really written this time -- unless you delete any old
version of the file. In particular, in preview mode, this should still be
called even if the file isn't going to be written to during the preview.

Ikiwiki uses this information to automatically clean up rendered files when
the page that rendered them goes away or is changed to no longer render
them. will_render also does a few important security checks.

#### `pagetype($)`

Given the name of a source file, returns the type of page it is, if it's
a type that ikiwiki knowns how to htmlize. Otherwise, returns undef.

#### `pagename($)`

Given the name of a source file, returns the name of the wiki page
that corresponds to that file.

#### `srcfile($;$)`

Given the name of a source file in the wiki, searches for the file in
the source directory and the underlay directories (most recently added
underlays first), and returns the full path to the first file found.

Normally srcfile will fail with an error message if the source file cannot
be found. The second parameter can be set to a true value to make it return
undef instead.

#### `add_underlay($)`

Adds a directory to the set of underlay directories that ikiwiki will
search for files.

If the directory name is not absolute, ikiwiki will assume it is in
the parent directory of the configured underlaydir.

#### `displaytime($;$)`

Given a time, formats it for display.

The optional second parameter is a strftime format to use to format the
time.

#### `gettext`

This is the standard gettext function, although slightly optimised.

#### `urlto($$;$)`

Construct a relative url to the first parameter from the page named by the
second. The first parameter can be either a page name, or some other
destination file, as registered by `will_render`.

If the third parameter is passed and is true, an absolute url will be
constructed instead of the default relative url.

#### `targetpage($$)`

Passed a page and an extension, returns the filename that page will be
rendered to.

## Miscellaneous

### Internal use pages

Sometimes it's useful to put pages in the wiki without the overhead of
having them be rendered to individual html files. Such internal use pages
are collected together to form the RecentChanges page, for example.

To make an internal use page, register a filename extension that starts
with "_". Internal use pages cannot be edited with the web interface,
generally shouldn't contain wikilinks or preprocessor directives (use
either on them with extreme caution), and are not matched by regular
PageSpecs glob patterns, but instead only by a special `internal()`
[[ikiwiki/PageSpec]].

### RCS plugins

ikiwiki's support for [[revision_control_systems|rcs]] is also done via
plugins. See [[RCS_details|rcs/details]] for some more info.

RCS plugins must register a number of hooks. Each hook has type 'rcs', 
and the 'id' field is set to the name of the hook. For example:
        
        hook(type => "rcs", id => "rcs_update", call => \&rcs_update);
        hook(type => "rcs", id => "rcs_prepedit", call => \&rcs_prepedit);

#### `rcs_update()`

Updates the working directory with any remote changes.

#### `rcs_prepedit($)`

Is passed a file to prepare to edit. It can generate and return an arbitrary
token, that will be passed into `rcs_commit` when committing. For example,
it might return the current revision ID of the file, and use that
information later when merging changes.

#### `rcs_commit($$$;$$)`

Passed a file, message, token (from `rcs_prepedit`), user, and ip address.
Should try to commit the file. Returns `undef` on *success* and a version
of the page with the rcs's conflict markers on failure.

#### `rcs_commit_staged($$$)`

Passed a message, user, and ip address. Should commit all staged changes.
Returns undef on success, and an error message on failure.

Changes can be staged by calls to `rcs_add, `rcs_remove`, and
`rcs_rename`.

#### `rcs_add($)`

Adds the passed file to the archive. The filename is relative to the root
of the srcdir.

Note that this should not commit the new file, it should only
prepare for it to be committed when rcs_commit (or `rcs_commit_staged`) is
called. Note that the file may be in a new subdir that is not yet in
to version control; the subdir can be added if so.

#### `rcs_remove($)`

Remove a file. The filename is relative to the root of the srcdir.

Note that this should not commit the removal, it should only prepare for it
to be committed when `rcs_commit` (or `rcs_commit_staged`) is called. Note
that the new file may be in a new subdir that is not yet in version
control; the subdir can be added if so.

#### `rcs_rename($$)`

Rename a file. The filenames are relative to the root of the srcdir.

Note that this should not commit the rename, it should only
prepare it for when `rcs_commit` (or `rcs_commit_staged`) is called.
The new filename may be in a new subdir, that is not yet added to
version control. If so, the subdir will exist already, and should
be added to revision control.

#### `rcs_recentchanges($)`

Examine the RCS history and generate a list of recent changes.
The parameter is how many changes to return.

The data structure returned for each change is:

        {
                rev => # the RCSs id for this commit
                user => # name of user who made the change,
                committype => # either "web" or the name of the rcs,
                when => # time when the change was made,
                message => [
                        { line => "commit message line 1" },
                        { line => "commit message line 2" },
                        # etc,
                ],
                pages => [
                        {
                                page => # name of page changed,
                                diffurl => # optional url to a diff of changes
                        },
                        # repeat for each page changed in this commit,
                ],
        }

#### `rcs_diff($)`

The parameter is the rev from `rcs_recentchanges`.
Should return a list of lines of the diff (including \n) in list
context, and the whole diff in scalar context.

#### `rcs_getctime($)`

This is used to get the page creation time for a file from the RCS, by looking
it up in the history.

It's ok if this is not implemented, and throws an error.

### PageSpec plugins

It's also possible to write plugins that add new functions to
[[PageSpecs|ikiwiki/PageSpec]]. Such a plugin should add a function to the
IkiWiki::PageSpec package, that is named `match_foo`, where "foo()" is
how it will be accessed in a [[ikiwiki/PageSpec]]. The function will be passed
two parameters: The name of the page being matched, and the thing to match
against. It may also be passed additional, named parameters. It should return
a IkiWiki::SuccessReason object if the match succeeds, or an
IkiWiki::FailReason object if the match fails.

### Setup plugins

The ikiwiki setup file is loaded using a pluggable mechanism. If you look
at the top of a setup file, it starts with 'use IkiWiki::Setup::Standard',
and the rest of the file is passed to that module's import method.

It's possible to write other modules in the `IkiWiki::Setup::` namespace that
can be used to configure ikiwiki in different ways. These modules should,
when imported, populate `$IkiWiki::Setup::raw_setup` with a reference
to a hash containing all the config items. They should also implement a
`gendump` function.

By the way, to parse a ikiwiki setup file and populate `%config`, a
program just needs to do something like:
`use IkiWiki::Setup; IkiWiki::Setup::load($filename)`