DifferenceEngine: Autodetect if wikidiff2 is installed

If wikidiff2 is installed, use it for diffing without requiring
$wgExternalDiffEngine to be set to 'wikidiff2'.

Also add some extra sanity-checking by coercing all non-string values to
false and make sure the custom diff executable passes is_executable().

Change-Id: I32b670ebf613be9f1c034d65d006829a215614da