add success and fail status

[tjaartvdwalt]

Hi,

Thanks for a cool project.

I want to use slack-webhook-cli as part of my CI pipeline. It would be nice if it supported Slack's "success" and "failed" states. This creates a green or red bar next to your message, making it easier to distinguish between failed and successful jobs without having to read the text.

[anthonyringoet]

Long overdue answer but anyway: feel free to contribute.