Make coala a GitHub App

See https://github.com/marketplace/category/code-quality

I guess coala would be much more popular if it can be made a GitHub App.

This means coala needs to be run as a web service, deployed somewhere like Heroku (but it's not totally free hmmm....)

GitMate + coala actually do the same thing, but that means developers need to learn both GitMate and coala, and GitMate requires admin access, which sounds a bit risky to developers (that's because GitMate has other functionalities, but coala does not need that high-level permission).