Progress reporting

At the moment we don't give good feedback to the user when the build tool is building stuff. Ideally, we would show something like "Building dependencies 54/500" or something via progress reporting.

However this is pretty hard to do well since we just shell out to the build tool and don't have a proper interface. Nonetheless, a hacky implementation in terms of parsing the stdout or something would be really nice for users and so I think it's probably worth it.

We'd need two things:

1. An interface for progress reporting for cradles
2. An implementation of some kind of progress reporting for, say, cabal