Topic lines/dynamic room names

[mgree]

Would be great to have a way to indicate the "status" of a room. That is, if I start an informal chat room and then we start talking about topic X, it'd be great to have a way to communicate that to others who might want to join. Conventional ways of doing this are changing the room name or setting a special "topic" field.

Idea due to @hgoldstein95.