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?
There's some duplication between Github/Gitlab formatters/clients. But I can figure out if I want to refactor that in any way myself, don't worry about it :smile: