Add hook to comments_are_now_closed()

Will permit addons to override / change parameters for comment closure.

NOTE: Custom rules will only work where the appropriate addon is installed. For this reason, addons which allow custom comment closure rules should carefully consider implementation. One recommended practice would be to "close" comments immediately on object creation using the default rules (date/time based) and allow commenting only to logged in observers of the item's "owner_xchan".