code: func New(channelSecret, channelToken string, options ...ClientOption) (*Client, error) {
  	if channelSecret == "" {
  		return nil, errors.New("missing channel secret")
  	}
  	if channelToken == "" {
  		return nil, errors.New("missing channel access token")
  	}
  	c := &Client{
  		channelSecret: channelSecret,
  		channelToken:  channelToken,
  		httpClient:    http.DefaultClient,
  	}
  	for _, option := range options {
  		err := option(c)
  		if err != nil {
  			return nil, err
  		}
  	}
  	if c.endpointBase == nil {
  		u, err := url.ParseRequestURI(APIEndpointBase)
  		if err != nil {
  			return nil, err
  		}
  		c.endpointBase = u
  	}
  	return c, nil
  }
docs: // New returns a new bot client instance.