Websocket handshake handler

[MattCheely]

There doesn't seem to be a way with the current implementation to handle the initial websocket handshake, and do things like set the sub-protocol or reject the request prior to opening the websocket connection. I'm wondering if I've missed something, or if that actually isn't implemented?

If it hasn't been implemented, I have some ideas around how it might be tackled if you're open to a PR.

[mpenet]

Sure, I am interested! I actually have some early work going that direction in a stash I think, but feel free to open a PR with your take on it.