code: function create_oauth2_client () {
    var google_client_id = (OPTIONS.GOOGLE_CLIENT_ID) ? OPTIONS.GOOGLE_CLIENT_ID : process.env.GOOGLE_CLIENT_ID;
    var google_client_secret = (OPTIONS.GOOGLE_CLIENT_SECRET) ? OPTIONS.GOOGLE_CLIENT_SECRET : process.env.GOOGLE_CLIENT_SECRET;
    var base_url = (OPTIONS.BASE_URL) ? OPTIONS.BASE_URL : process.env.BASE_URL;
    oauth2_client = new OAuth2Client(
      google_client_id,
      google_client_secret,
      base_url + OPTIONS.REDIRECT_URL
    );
    return oauth2_client;
  }
docs: create_oauth2_client creates the OAuth2 client
  @param {Object} options - the options passed into the plugin
  @returns {Object} oauth2_client - the Google OAuth2 client