Make `gitlab-rulez` installable with `pip`

This adds very basic integration into the PIP infrastructure. See the readme for usage instructions. Apart from that, you verify installation right now using

pip install git+https://gitlab.apertis.org/ulrich.eckhardt/gitlab-rulez.git@feature/pip-installable-package

Notes

• I noticed you don't have any merge commits in this repo. If you want to keep this, it would probably make sense to squash these five commits. I've kept them separate to make them easier to review.
• This will definitely clash with my other open merge request. If in doubt, merge this one and I'll rebasefix the other.
• This is not integrated into the PIP package registry, so you need to provide the full URL to pip! In particular something like pip install gitlab-rulez will not work! This can be made to work in the future though, but that would also require proper versioning.
• The initial gitlab-rulez script was split off into a module that could be reused but also re-organized under the hood. The original script still exists, but could at some point be removed. When installing via pip, a similar wrapper script around the module is created already.