Add GitLab CI configuration

This took way longer than expected, in part due to the linter.

I am quite happy with how it is right now, I'll change the linter repository from my fork to upstream once my fix gets in.

I hesitated a bit making the linter a submodule. What do you think? For now, the commit is pinned inside the CI script.

As usual, please make sure the merge commit has a meaningful title, I prefer it that way for git log --first-parent 😉

Resolves #22 (closed)