Go-github: Support new webhook events and actions

[gmlewis]

GitHub Developer API announcement:
https://developer.github.com/changes/2019-04-18-new-webhook-events-and-actions/

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 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!

[joshuabezaleel]

Hi @gmlewis ! Can I take a shot on this? 🙂

[gmlewis]

Sure, @joshuabezaleel! Thank you!
It is yours.

[joshuabezaleel]

Hi @gmlewis , how do we have to write/generate for the github-accessors.go? Or it will be automatically generated by the gen-accessors.go?

[gmlewis]

Check out CONTRIBUTING.md. It has all the details.