Let caller of setupWSConnection create their own callback function

[canadaduane]

This is an up-to-date PR that replaces #61

Nothing has changed other than rebasing onto master. I've tested 2 of the 3 cases mentioned by @dmonad in https://github.com/yjs/y-websocket/pull/61#issuecomment-822020739

The untested case is the CALLBACK_URL scenario, because I don't have such a callback server set up. If anyone could test that this works for them in the "regular" scenario, that would be appreciated!