Adds GitLab formatter

Closes https://github.com/mmozuras/pronto/issues/25

We've been running this in production for the past few months and it has been working smoothly.

This will be released with GitLat v7.5.0 tomorrow. This does wait on an update on https://github.com/NARKOZ/gitlab/pull/88 so it can not be merged just yet.

Could you review the code and let me know if anything should be changed?
Could you split changes to the README to a separate pull request? I'll merge it in after releasing a new version. Otherwise, it could get a little confusing with people asking "why doesn't it work if it's in README?" :smile: