allow mirror also submodules

[gocarlos]

projects like protobuf or grpc have a lot of submodules which are needed at compile time...

It would be great if this tool could also mirror the submodules with e.g. a regexpression -> submodules under github.com/mygroup/my_project -> mygitlab.com/mymirror/mygroup_my_project

Eventually we would need to give this tool API access to a gitlab instance to create repos automatically

cc/ @max-wittig