Improve source type validation

See merge request !6

This implementation will raise an error if more than one source type is provided for a given item. Previous implementation was raising an error if all source type were provided (this is valid with two source types but will break if we add a third source type).
This implementation also detect lack of source type.

Add some unit test.

Add comments in source code to ease understanding (added by someone unfamiliar with Python :) and renamed variable for readability.

Try to clarify the git-mirror capabilities

See merge request !3

Keep repository deleted or archived from GitHub in GitLab

See merge request !2

git-mirror handle the synchronization of a whole GitHub organization to GitLab.
It is possible to disable the synchronization of previously synchronized GitLab repository by adding: `skip: true` to the GitLab repository description.

As we want to archive all project content we don't want to remove a repository that is no longer part of the project (because it has been archived or deleted from GitHub). We rather want to disable the synchronization to GitLab and archive it in GitLab.

remove all print() call from mirrorIt

- Print the tool's stderr only when no exception occured.