This allows plugins and other extensions to directly extend MUC with additional
protocol support, such as support for a Conference Focus on the MUC room JID,
and so on.

* addIQHandler adds a new IQHandler to services and rooms.
* addExtraFeature is the existing function, exposed at the Interface. These
additional features are now shown for both rooms and services.
* addExtraIdentity is the existing function exposed unchanged.