Support for unix sockets

Websockets has a support for unix sockets with a url like ws+unix:///path/to/file, but Socket.io doesn't seem to support it.

The server:

const path = './server.sock';

var app = require('http').createServer(function(req, res) {
    console.log('Request for ' + req.url);
    res.statusCode = 200;
    res.setHeader('Content-Type', 'text/plain');
    res.end('Hello World\n');
});

var io = require('socket.io')(app);

io.on('connection', function (socket) {
    socket.emit('news', { hello: 'world' });
    socket.on('my other event', function (data) {
        console.log(data);
    });
});

app.listen(path, function() {
    console.log(`Server running at ${path}`);
});

...and client:

const path = './server.sock';
var socket = require('socket.io-client')('ws+unix://' + require('path').resolve(path));
socket.on('connect', function(){
        console.log('connect');
});
socket.on('event', function(data){
        console.log('event: ', data);
});
socket.on('disconnect', function(){
        console.log('disconnect');
});

...but client doesn't connect...

Start by tracing how the Socket.IO client parses connection URLs and how the Node.js server and client create their underlying connections. Reproduce the provided server.sock example, then add support and tests showing that a ws+unix URL connects successfully and preserves existing URL behavior.