includes/Linker.php: Added hook for "Media:" links

includes/Linker.php: Added hook for "Media:" links

The static method 'Linker::makeMediaLinkFile' produced a HTML string
without usage of MediaWiki 'Html' class. It also lacked a hook to allow
modifications of the output HTML.

I altered the implementation and added a hook that provides a signature
and behaviour similar to the existing 'LinkerMakeExternalLink' and
'LinkBegin'/'LinkEnd' hooks. It provides all available context information
and allows modification of single attributes or the output HTML as a
whole. I have updated the 'docs/hooks.txt' file to provide proper
documentation.

Change-Id: I6d7769298a4ca6cbbf807fcebb91fb0d2222f8d8