Selectively redirect submodules to the cached repo

If Kochiku is asked to build a branch that contains a new submodule, Kochiku will redirect the origin of that submodule to the "cached" repo but that submodule will not be present inside the cached repo and the build will fail. Handle this case by only redirecting the submodules that are present in the cached repo and perform a full clone of the rest. The repeated full clone is inefficient but at least it works.

Resolves #25 

To: @square/kochiku-contributors 
cc: @rostyboost