code: func New(channelSecret, channelToken string) (*WebhookHandler, error) {
  	if channelSecret == "" {
  		return nil, errors.New("missing channel secret")
  	}
  	if channelToken == "" {
  		return nil, errors.New("missing channel access token")
  	}
  	h := &WebhookHandler{
  		channelSecret: channelSecret,
  		channelToken:  channelToken,
  	}
  	return h, nil
  }
docs: // New returns a new WebhookHandler instance.