Organize existing git repositories similar to git-grab

[Byron]

When installing ein with cargo install gitoxide, one can run ein tools organize to get a listing of how repositories would be moved if --execute was specified.

It could help to organize a pre-existing loose structure into the one proposed by git-grab, and from there git-grab could be used to maintain structure on any clone.

In any case, I will be using git-grab from now on as it's easier than git clone + ein t organize -e.