Add support for gitlab-lint

[scottrbaxter]

Name: gitlab-lint URL: https://github.com/elijah-roberts/gitlab-lint

Linter for .gitlab-ci.yml files, using the gitlab api.

The cli version (installed via python3 -m pip install -U gitlab_lint) has been very useful, and I believe would make a great addition to ale.

[thegcat]

Duplicates #843

[thegcat]

I think #3042 would be a great addition. What would be needed to get this going?

[hsanson]

#3042 need someone to review or at least test it.