submodule init: initialize active submodules

submodule init: initialize active submodules

Teach `submodule init` to initialize submodules which have been
configured to be active by setting 'submodule.active' with a pathspec.

Now if no path arguments are given and 'submodule.active' is configured,
`init` will initialize all submodules which have been configured to be
active.  If no path arguments are given and 'submodule.active' is not
configured, then `init` will retain the old behavior of initializing all
submodules.

This allows users to record more complex patterns as it saves retyping
them whenever you invoke update.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>