Support custom target addresses

[Travis Ralston]

In addition to supporting the room target, we should also allow custom email addresses for each room. This is for mailing list mirrors that use redirects - they don't provide any information we can parse out to find a room ID.

For example, I should be able to bind [e-mail] to !roomid:domain.com. If the bot receives email targeted for [e-mail], it should post in the mapped room.