code: func (api *Client) PostMessage(channelID string, options ...MsgOption) (string, string, error) {
  	respChannel, respTimestamp, _, err := api.SendMessageContext(
  		context.Background(),
  		channelID,
  		MsgOptionPost(),
  		MsgOptionCompose(options...),
  	)
  	return respChannel, respTimestamp, err
  }
docs: // PostMessage sends a message to a channel.
  // Message is escaped by default according to https://api.slack.com/docs/formatting
  // Use http://davestevens.github.io/slack-message-builder/ to help crafting your message.