Optional "@-mention invites users into public rooms" semantics

A feature rocketchat and a few other systems have is that if you @-mention someone in a public room, they automagically get invited into the room, given the @-mention almost has twitteresque "i'm sending this message to you" semantics.

Having this as a configurable option per-room (or per-client) might be a helpful way of mitigating the problem of absent Matrix users getting kicked from IRC rooms, as per https://github.com/matrix-org/matrix-appservice-irc/issues/388#issuecomment-355745510