Add websocket support with Gun

[ajuvercr]

Expand cowboy routing so /ws[...] are redirected to a Gun Websocket handler.
To facilitate this a ws macro is added to matcher that sets up 'route' by generating an ID.
This ID is a query parameter appended to the redirect URI and mapped to backend host, port and path.

Example:

dispatcher.ex

  match "/ws2" do
    ws(conn, "ws://localhost:7999")
  end

Route '/ws2/ to localhost:7999 with websockets.

Note: following redirects is not required in the websockets specs, and most browsers don't support this.

Example workarounds:

async function createRedirectedWebsocket(url) {
    // This prints an error to the console due to unexpected upgrade response
    const resp = await fetch(url);
    const ws_url = resp.url.replace(/^http/, 'ws');
    return new WebSocket(ws_url);
}

With the 'ws' npm package it is possible to set a followRedirects flag.

const WebSocket = require('ws')
const ws = new WebSocket('ws://localhost/ws2', options={'followRedirects': true});

[madnificent]

Thanks for the PR!

Do WebSockets always need another port to operate on or can they be routed through? I'm missing that from the description here. If you'd update the README in the PR, I would consider this to be experimental functionality.

[ajuvercr]

mu-dispatcher only uses one port, all requests to a URL starting like /ws/[...] are redirected to a Websocket handler.