PR proposal - support rooms

[benallfree]

I see several requests for rooms (#1134 #1237 #1083). I would like to propose a simple approach:

Client changes

Support a PeerOptions.room field. When present, it will send an additional querystring param /:key/id?room=<room> or X-PeerJS-Room: <room> header.

const client = new Peer({ room: `myRoomName` })

Server changes:

Look for a querystring room param on /:key/id?room=<room>. Store room-specific peers in a separate map.

Modify allow_discovery: "room" to allow discovery only if a room querystring param is specified

Thoughts? Happy to send a PR.