Allow mirroring on different groups

Hi and thanks for this project.

If I well understand the documentation, we have to create a "Mirrors" group and set up project to mirror that will be auto added on this group.

Could be very useful to have the possibility to set group when adding a mirror.

For example:

./add_mirror.sh --git --project-group samrocketman --project-name gitlab-mirrors --mirror https://github.com/samrocketman/gitlab-mirrors.git

Thanks.