Support the CheckSuite + CheckRun webhook events

[AlistairB]

The checks API + events are pretty new to github. This library does not currently support the following events.

• https://developer.github.com/v3/activity/events/types/#checksuiteevent
• https://developer.github.com/v3/activity/events/types/#checkrunevent

I am working on a PR to add these. This is the corresponding issue.

Note: The checks API is in preview. Are you still happy to add support?