Add extension-specific types functionality to ModelRegistry

Extension-specific types are RDFa types (or type regexes) that are
registered with the ModelRegistry separately. If an element has a type
that is extension-specific, then that element can only be matched by a
rule that asserts one of its extension-specific types.

For MediaWiki, we would call
ve.dm.modelRegistry.registerExtensionSpecificType(/^mw:/ ) .
So then an element like <span typeof="mw:foobar"> would either match a
rule specifically for mw:foobar, if one exists, or no rule at all; even
the rule for <span> would not match. The consequence of this is that
elements with unrecognized mw:-prefixed RDFa types are alienated.

Change-Id: Ia8ab1fe5dffb9f813689324372a168e8e4a3e0bc
Merge "Add extension-specific types functionality to ModelRegistry"