Add support for regex path based selection

[zoonage]

Use case: We were trying to configure a set of pipelines that are sometimes working in the top level of the code base and sometimes not. We are using set_pipeline and a template pipeline in Concourse to configure these.

Because neither "" (matches nothing) "*" (only matches top level directory changes) match we are unable to match the entire fs tree of the PR. This causes certain builds to not trigger. I've updated the documentation for paths to reflect that this isn't using glob syntax and is using path/filepath.Match syntax

This change adds support for identifying paths using regexes. This solves our use case by allowing us to use ^. for anything in the top level and path/to/subproject/^. for anything else.