code: func MsgOptionUnfurl(timestamp string, unfurls map[string]Attachment) MsgOption {
  	return func(config *sendConfig) error {
  		config.endpoint = config.apiurl + string(chatUnfurl)
  		config.values.Add("ts", timestamp)
  		unfurlsStr, err := json.Marshal(unfurls)
  		if err == nil {
  			config.values.Add("unfurls", string(unfurlsStr))
  		}
  		return err
  	}
  }
docs: // MsgOptionUnfurl unfurls a message based on the timestamp.