Implement support for JSON Pointer (RFC 6901) in "sops --set"

sops --set uses a json encoded string to identify which key/value to write/change.
This json encoded string is used when modifying json files or yaml files.

Some struture-aware diff tools create "patch files" that contain JSON Pointer to reference the value that should be added, deleted or changed. One tool of that kind: https://github.com/swaggest/json-diff

Example patch file content (where "path" is the JSON Pointer):

[
    {"value":4,"op":"test","path":"/key1/0"},
    {"value":5,"op":"replace","path":"/key1/0"},
    {"op":"remove","path":"/key2"},
    {"op":"remove","path":"/key3/sub0"},
    {"value":"a","op":"test","path":"/key3/sub1"},
    {"value":"c","op":"replace","path":"/key3/sub1"},
    {"value":"b","op":"test","path":"/key3/sub2"},
    {"value":false,"op":"replace","path":"/key3/sub2"},
    {"value":0,"op":"add","path":"/key3/sub3"},
    {"value":true,"op":"add","path":"/key4/0/subs/2/add"},
    {"op":"remove","path":"/key4/1/b"},
    {"value":false,"op":"add","path":"/key4/1/c"},
    {"value":1,"op":"add","path":"/key4/2/c"},
    {"value":"wat","op":"add","path":"/key5"}
]

To apply such a patch value-by-value via sops --set '' it is cumbersom to convert a JSON Pointer to a --set-compatible json encoded string. Please add support for JSON Pointer.

Instead of:
sops --set '["key3"][sub0] true'
it should be possible to write:
sops --setPointer '/key3/sub0 true'

This feature would help to implement a "sops auto merge conflict resolver".
https://github.com/mozilla/sops/issues/52

Start at the CLI handling for sops --set and trace how its JSON-encoded paths are parsed and applied. Compare that behavior with RFC 6901 paths, using the examples in the issue as cases; done means sops --setPointer '/key3/sub0 true' and equivalent pointer paths can update JSON and YAML values.