Support specifying subprotocol

[aymanosman]

Example javascript code

new WebSocket("ws://localhost:5280/ws-xmpp", ["xmpp"])

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[liamcurry]

Seconding this. Apollo's GraphQL WebSocket server requires the protocol to be specified by the client, so it can't be used with Elm at the moment due to this issue.