code: func New(mo Options) *Messenger {
  	if mo.Mux == nil {
  		mo.Mux = http.NewServeMux()
  	}
  
  	m := &Messenger{
  		mux:       mo.Mux,
  		token:     mo.Token,
  		verify:    mo.Verify,
  		appSecret: mo.AppSecret,
  	}
  
  	if mo.WebhookURL == "" {
  		mo.WebhookURL = "/"
  	}
  
  	m.verifyHandler = newVerifyHandler(mo.VerifyToken)
  	m.mux.HandleFunc(mo.WebhookURL, m.handle)
  
  	return m
  }
docs: // New creates a new Messenger. You pass in Options in order to affect settings.