localstyle: New plugin, allows overrding the toplevel local.css with one that is...

localstyle: New plugin, allows overrding the toplevel local.css with one that is closer to a page.

I chose not to have it override style.css, because style.css is not really
intended to be edited; the one from the underlay is intended to be used as
a base that local.css overrides.

I chose to use a plugin rather than changing the default behavior, both
because I didn't want to have to worry about possibly breaking backwards
compatability (though this seems unlikely), and because it seemed cleaner
to not include style template parameters in the main page template code.

I suppose someone might want a way to not override the toplevel
local.css, but instead include it as well as foo/local.css. Probably the
best way to do that would be to have foo/local.css @import ../local.css
(modulo browser compatability issues). Alternatively, edit page.tmpl
to always include the toplevel local.css, or swap out this plugin for
another one.