HTTPS support, Promise API, modern ES syntax

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

.eslintrc.js

@@ -0,0 +1,21 @@
+module.exports = {
+  parser: 'babel-eslint',
+  extends: ['airbnb', 'prettier', 'plugin:jest/recommended'],
+  plugins: ['prettier', 'jest'],
+  env: {
+    'jest/globals': true,
+  },
+  rules: {
+    'prettier/prettier': [
+      'warn',
+      {
+        printWidth: 100,
+        tabWidth: 2,
+        bracketSpacing: true,
+        trailingComma: 'es5',
+        singleQuote: true,
+        jsxBracketSameLine: false,
+      },
+    ],
+  },
+}