Evaluate includes within exceptions

[tuckermcclure]

Previously, "exceptions" in the include process were treated as already-complete values. This PR allows them to also include files, which can be a powerful pattern. Consider:

include:
  source: roses.yaml
  except:
    - path: rosaceae.malus # Update this value from literals below.
      value:
        - domestica
        - fusca
    - path: rosaceae.sorbus # Update this value with whatever's in sortbus_list.yaml.
      value:
        include: sorbus_list.yaml

There's no ambiguity in the syntax; it's exactly the same as everywhere else.

I added a test for this behavior. It also pointed out a small bug: the included files were previously assumed to return dictionaries, but they can also return arrays or values. These cases are handled explicitly now and also tested.

I tested this on a downstream project.