Support in Submodule Git Clone

[RyanSiu1995]

Thank you for the great tool in protobuf development!
I have used the prototool as the change break detection.
But in my repository, I use the git submodule to import different proto files into my application repository.
Therefore, it is good if we could have such flag to enable the submodule initialization.
For example like this,

prototool break check --git-branch master --submodule true

And the thing it will do is just the following after the cloning.

git submodule init
git submodule update