Allow for Shallow Clone command in 2.0
As per https://github.com/circleci/circleci-docs/issues/2040#issuecomment-368139737
It would be nice for users to have a way to shallow clone. Currently, we do `checkout` to checkout. It would be nice to be able to pass a depth into that like `shallowClone:10` and then use the number `10` as the depth for the clone. The PR linked has a modified version of the existing script that works.