code: function Strategy(options, verify) {
    if (typeof options == 'function') {
      verify = options;
      options = {};
    }
    if (!verify) { throw new TypeError('AuthTokenStrategy requires a verify callback'); }
    
    this._authtokenField = options.authtokenField || 'token';
    this._checkFields = options.checkFields || [];
    if (typeof(this._checkFields) === 'string') {
      this._checkFields = this._checkFields.split(',');
    }
    
    passport.Strategy.call(this);
    this.name = 'authtoken';
    this._verify = verify;
    this._passReqToCallback = options.passReqToCallback;
  }
docs: `Strategy` constructor.
  
  The authtoken authentication strategy authenticates requests based on the
  credentials submitted through an API-based login.
  
  Applications must supply a `verify` callback which accepts `token` and
  others credentials, and then calls the `done` callback supplying a
  `user`, which should be set to `false` if the credentials are not valid.
  If an exception occured, `err` should be set.
  
  Optionally, `options` can be used to change the fields in which the
  credentials are found.
  
  Options:
  - `authtokenField`  field name where the authtoken is found, defaults to _token_
  - `checkFields`  field names array where the other phrase are found, defaults to _[]_
  - `passReqToCallback`  when `true`, `req` is the first argument to the verify callback (default: `false`)
  
  Examples:
  
  passport.use(new AuthTokenStrategy(
  function(token, other1, other2, done) {
  User.findOne({ token: token, other1: other1 }, function (err, user) {
  done(err, user);
  });
  }
  ));
  
  @param {Object} options
  @param {Function} verify
  @api public