mkdocs-gitlab-plugin improvements

mkdocs-gitlab-plugin GitLab has minor issues:

• Does not transform reference to "new" GitLab URL: domain.tld/$group/$project/-/$type/$number. The /-/ in the middle is missing. This is currently not an issue for issues, merge_requests, snippet since GitLab is adding the /-/. But for milestones, it is not redirected and end-up on a 404 not found. Thought, I can’t test epics since we have a self-hosted GitLab. Anyway, should be fix.
• had to find the name of the plug-in name in setup.py: gitlab_links, which wasn’t obviously stated in the documentation.
• Would be great to being able to link other projects such as DuniterPy

Contribute

• Contact its author on how to integrate these two first minor fixes:
  • https://cnrs.magiraud.org/ mathieu.giraud@univ-lille.fr
• He integrate my changes, and publish new release on PyPI
• Otherwise, I have to fork it.