Support the older <youtube>video ID</youtube> syntax.

This is a follow-up to https://gerrit.wikimedia.org/r/#/c/101672/

Reported on the ShoutWiki Bugzilla: http://bugzilla.shoutwiki.com/show_bug.cgi?id=249#c2

Change-Id: I8e2ae0d200e286de1478bf294cd02bc569b056a8