[webhook] Allow passing options to HTTPS server

Feature:

  We shall allow passing more options to the HTTP server,
  in `https.createServer()`.

  We are using a new property, `https`, to avoid any namespace
  collisions with our own options.

  `options.key`, `options.cert` and `options.pfx` are convenient
  options, in that they allow the user to provide paths to the
  corresponding files, which are read automatically,
  though synchronously!

Implementation:

  * completely backwards-compatible
  * all changes are being tested, except `options.pfx`

References:

  * Pass `ca` prop to https.createServer(): https://github.com/yagop/node-telegram-bot-api/pull/17

test/utils.js

@@ -37,6 +37,7 @@ exports = module.exports = {
    * @param  {Object} [options]
    * @param  {String} [options.method=POST] Method to use
    * @param  {Object} [options.message] Message to send. Default to a generic text message
+   * @param  {Boolean} [options.https=false] Use https
    * @return {Promise}
    */
   sendWebHookMessage,
@@ -136,7 +137,8 @@ function hasOpenWebHook(port, reverse) {
 function sendWebHookMessage(port, token, options = {}) {
   assert.ok(port);
   assert.ok(token);
-  const url = `http://127.0.0.1:${port}/bot${token}`;
+  const protocol = options.https ? 'https' : 'http';
+  const url = `${protocol}://127.0.0.1:${port}/bot${token}`;
   return request({
     url,
     method: options.method || 'POST',