support github checks API

[geekflyer]

Hi,

thanks for this awesome tool!
One thing i'd like to request is for the gh-app to support posting the detailed status results to the github checks API (https://developer.github.com/v3/checks/) instead of posting it as a comment.

The github comments just tend to be a bit spammy since each comment causes an email notification. I think putting the details of the check results into the checks api would be a nicer, more github native way of this displaying this information.

Another advantage of using this API is that it would allow the app to distinguish the result into actual failure (i.e. something broke during the screenshot process) and a screenshot diff by using the conclusion field of github checks.
For example I'd suggest to use action required instead of failure as conclusion when screenshot changes were found since it better distinguishes the fact that a screenshot diff doesn't really mean failure but rather just means that someone has to review and approve it (more like action required).
Here's how these different conclusions are displayed in github:
https://developer.github.com/assets/images/check_runs.png

Thanks for your consideration.