feat: add --skip-submodules argument

[ypid]

One could argue that this should even be the default because the submodule is managed by containing repository. If the submodule is dirty, the containing repository is dirty as well. Plus the submodule might very well not be on a branch like master but on a specific git commit and that is fine.

But better not make it the default as it would change the behavior. When submodules annoy clustergit users, they can now use the --skip-submodules argument