Feature request: support monorepos

So, I'm forced by my coworkers to live in a mono-repo.
This makes versioning of particular libraries, applications, or what-have-you individually hard.

But doing so would still be very useful, especially for things like FaaS code.

As such, it'd be great if PBR could support doing it's calculations based on two things:
1) A set of "watched" folders from the root of the VCS repository (i.e, I might watch `lib/lambda_base` and `lambdas/foo` for the foo lambda
2) a prefix in the tag. So rather than just having one tag to rule them all, I might have lambda-foo:v1.0.4

you might also need to use the tag prefix in the commit messages to do your calaculation.

Thanks for the consideration!

sean