review 'append' op

[mikemccabe]

I added an 'append' op to support a use-case in my organization. I think this is a case where the spec was lacking support for a common real-world use. However, it needs review.

From local_tests.json:

{ "comment": "append",
  "doc": [1, 2, 3, 4],
  "patch": [{"op": "append", "path": "/-", "value":[5, 6, 7, 8]}],
  "expected": [1, 2, 3, 4, 5, 6, 7, 8] },