HTTPS support, Promise API, modern ES syntax

* Add support for tunneling a local HTTPS server.
* Return a Promise from localtunnel.

localtunnel.js

@@ -0,0 +1,14 @@
+const Tunnel = require('./lib/Tunnel');
+
+module.exports = function localtunnel(arg1, arg2, arg3) {
+  const options = typeof arg1 === 'object' ? arg1 : { ...arg2, port: arg1 };
+  const callback = typeof arg1 === 'object' ? arg2 : arg3;
+  const client = new Tunnel(options);
+  if (callback) {
+    client.open(err => (err ? callback(err) : callback(null, client)));
+    return client;
+  }
+  return new Promise((resolve, reject) =>
+    client.open(err => (err ? reject(err) : resolve(client)))
+  );
+};