Add support for connecting to files/unix sockets.

Closes mqttjs#1040.

README.md

@@ -164,7 +164,7 @@ Connects to the broker specified by the given url and options and
 returns a [Client](#client).
 
 The URL can be on the following protocols: 'mqtt', 'mqtts', 'tcp',
-'tls', 'ws', 'wss'. The URL can also be an object as returned by
+'tls', 'ws', 'wss', 'file' (e.g. unix socket). The URL can also be an object as returned by
 [`URL.parse()`](http://nodejs.org/api/url.html#url_url_parse_urlstr_parsequerystring_slashesdenotehost),
 in that case the two objects are merged, i.e. you can pass a single
 object with both the URL and the connect options.

lib/connect/file.js

@@ -0,0 +1,8 @@
+'use strict'
+var net = require('net')
+
+function buildBuilder (client, opts) {
+  return net.createConnection(opts.pathname)
+}
+
+module.exports = buildBuilder

lib/connect/index.js

@@ -12,6 +12,7 @@ if (process.title !== 'browser') {
   protocols.ssl = require('./tls')
   protocols.tls = require('./tls')
   protocols.mqtts = require('./tls')
+  protocols.file = require('./file')
 } else {
   protocols.wx = require('./wx')
   protocols.wxs = require('./wx')