GitHub checks support

[SquidDev]

It would be nice to have integration with GitHub checks. We could have a --github-checks flag on Lint, which uses GITHUB_TOKEN and GITHUB_REF to send an API request.

There's a nice example of a Robocop linter doing the same thing.

We could also use requested_actions to fix the issues, which would be neat.

• [x] Basic CoHttp support
• [ ] Node.js support for http requests. We might need to make the report method return Lwt.t (and so make the toplevel responsible for executing async code).
• [ ] Support for fixing and committing. I suspect this could be done entirely in bash, but want to try out before deploying anywhere.

[SquidDev]

Having looked into this a little more and after running this on CC: Tweaked, the current system isn't really suitable. GITHUB_TOKEN cannot be used to submit checks from forked repositories (as it has read-only permissions), and so illuaminate breaks.

I think the solution here is to strip out --github support and build a separate executable which just contains the linters and GitHub communication. We can run the linter from an external service.

[SquidDev]

We removed GH support in 8c2ad23fa8569062e7c156a04b26700b5a78f37f. For CC:T, I've ended up registering a problem matcher instead, which works rather well.

I think I'm going to close this, as I don't think running an external service for this is worth it.