HTTPS support, Promise API, modern ES syntax

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

lib/HeaderHostTransformer.js

@@ -1,39 +1,23 @@
-var stream = require('stream');
-var util = require('util');
+const { Transform } = require('stream');
 
-var Transform = stream.Transform;
+class HeaderHostTransformer extends Transform {
+  constructor(opts = {}) {
+    super(opts);
+    this.host = opts.host || 'localhost';
+    this.replaced = false;
+  }
 
-var HeaderHostTransformer = function(opts) {
-    if (!(this instanceof HeaderHostTransformer)) {
-        return new HeaderHostTransformer(opts);
-    }
-
-    opts = opts || {}
-    Transform.call(this, opts);
-
-    var self = this;
-    self.host = opts.host || 'localhost';
-    self.replaced = false;
+  _transform(data, encoding, callback) {
+    callback(
+      null,
+      this.replaced // after replacing the first instance of the Host header we just become a regular passthrough
+        ? data
+        : data.toString().replace(/(\r\n[Hh]ost: )\S+/, (match, $1) => {
+            this.replaced = true;
+            return $1 + this.host;
+          })
+    );
+  }
 }
-util.inherits(HeaderHostTransformer, Transform);
-
-HeaderHostTransformer.prototype._transform = function (chunk, enc, cb) {
-    var self = this;
-
-    // after replacing the first instance of the Host header
-    // we just become a regular passthrough
-    if (!self.replaced) {
-        chunk = chunk.toString();
-        self.push(chunk.replace(/(\r\n[Hh]ost: )\S+/, function(match, $1) {
-            self.replaced = true;
-            return $1 + self.host;
-        }));
-    }
-    else {
-        self.push(chunk);
-    }
-
-    cb();
-};
 
 module.exports = HeaderHostTransformer;