open() {
    if (this.isOpen()) {
      return Promise.resolve();
    }
    return new Promise((resolve, reject) => {
      this._webServer.listen(this.options.port, this.options.host, () => {
        debug('WebHook listening on port %s', this.options.port);
        this._open = true;
        return resolve();
      });

      this._webServer.once('error', (err) => {
        reject(err);
      });
    });
  }
Open WebHook by listening on the port
@return {Promise}