Adding SpecialSearchResultsPrepend/Append hooks

Adding SpecialSearchResultsPrepend/Append hooks

These hooks allow you to prepend, append or replace the search results
in Special:Search.

SpecialSearchResultsPrepend is executed immediately before the results
HTML begin to output.  It can be used to include an external search
provider like Google CSE, for example.  If the hook returns true, the
MediaWiki search results continue to output.  If returned false, they
are suppressed.

SpecialSearchResultsAppend is executed immediately after the last
results HTML (including search navigation) are output.  This hook is of
limited use since it will not be run in certain circumstances (there are
several points in the previous output logic where the function returns),
but is included for completeness.

Change-Id: Ib38433141979de999cdcab5eca26ba5416331aaa