code: func TokenHandler(cb AuthTokenHandler) Option {
  	return func(o *Options) error {
  		if o.Token != "" {
  			return ErrTokenAlreadySet
  		}
  		o.TokenHandler = cb
  		return nil
  	}
  }
docs: // TokenHandler is an Option to set the token handler to use
  // when a token is not included directly in the URLs
  // and when a token is not set.