Add hook_user_config

This is what our fork has been using to get per-recip config values for some years.  There's been some question as to whether the hook_config API could be extended instead.  I haven't written any unit tests yet, but will do so if we don't decide I'm rewriting the code first.
Remove. The rise of wicked fast key-value stores that can be both local and centralized making doing this in perl less desirable.