code: func (rtm *RTM) NewOutgoingMessage(text string, channelID string, options ...RTMsgOption) *OutgoingMessage {
  	id := rtm.idGen.Next()
  	msg := OutgoingMessage{
  		ID:      id,
  		Type:    "message",
  		Channel: channelID,
  		Text:    text,
  	}
  	for _, option := range options {
  		option(&msg)
  	}
  	return &msg
  }
docs: // NewOutgoingMessage prepares an OutgoingMessage that the user can
  // use to send a message. Use this function to properly set the
  // messageID.