Allow exceptions to target vector entries

[tuckermcclure]

From the updated readme:

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For example, trees.yaml might provide shared data:

trees:
  - scientific_name: Arbutus unedo
    common_name: strawberry tree
  - scientific_name: Arbutus menziesii
    common_name: Pacific madrone
notes: Needs review.

Another file can include it and overwrite selected values:

include:
  source: trees.yaml
  except:
    - path: trees[2].common_name
      value: madrona
    - path: notes
      value: Reviewed.

Loading the second file produces the included data with the second tree's common name changed and the top-level notes value replaced. The trees[2] path uses Julia-style 1-based indexing.

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