Split out core functionality

To allow for greater configurability and reuse, I'd like to split the following pieces out into separate packages:

* The `src/linters/` directory into a new `@humanmade/linter-core` package
* `src/format.js` and `src/hooks.js` into a new `@humanmade/linter-github` package (and maybe some other bits too)

Ideally, I'd like to switch this repo over to a Lerna repo so we don't have to faff about with synchronising packages everywhere, and so that this is basically just a minor internal refactor. I'll need to test that this is working properly with our complex AWS deployment system though.

@mikeselander before I start on this, any objections?