Notification support

[donny-dont]

Would be great if maintainers were notified through Slack when a pull request they need to review is opened.

[samber]

I don't know the work that have been made on this, but I would suggest that notifying through Hubot would allow to connect to every Slack-like providers !

[bradrydzewski]

There hasn't been much work one notifications. I have created the interface to define how lgtm can invoke notification services: https://github.com/lgtmco/lgtm/blob/master/notifier/sender.go

I envision having a small number of built-in notification capabilities such as:

• github notification. comment on the pull request and @ mention the reviewers
• http webhook notification

The http webhook should be enough to let teams come up with any sort of custom notification workflow they want using which ever services they prefer.