Go-github: Support new Webhook Event for GitHub Apps using User-to-Server tokens

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2018-07-25-github-app-authorization-event/

This would be a great PR for any new contributor to this repo or a new Go developer.
All contributions are greatly appreciated!

Feel free to volunteer for any issue, and we can send you an invite to contribute to the repo (which you then accept after you enable two-factor authentication) and the issue can be assigned to you so that others don't attempt to duplicate the work.

Please check out our CONTRIBUTING.md guide to get started.

Thank you!

[sanketpayghan]

Will work on this

[MorrisLaw]

Is this still being worked on? @sanketpayghan @gmlewis I can pick it up if needed

[gmlewis]

Thank you, @MorrisLaw... I think it is safe to say that you can take it over.