Add "extended" file metadata to API

Add "extended" file metadata to API

Part of the point of this, is to add a hook to allow extensions to add
their own metadata (I intend to create a companion extension to parse
Commons description pages - I5e6bc45f9751)

It's hoped that this would provide a simple system to get file metadata,
and would be able to return information on any wiki (even without any
extensions installed to provide additional information). So it could
fallback to exif data, if there's no better source of information for
the file available. It's also meant to be done in such a way that, in
the future, when Wikibase is deployed on Commons, it could be integrated
without too much fuss.

marktraceur changed this patch to be a little less heavy-handed: it will
now return unformatted data by default from the API, but there's also a
method for formatting that data in the way that bawolff originally
intended. I'm still trying to figure out if there's much use in that
method, but for now it's not particularly useful.

Change-Id: I77303d8e535fc1c42e14cfb853814e5c434a81ec