Support dynamic lookups into other fields

[g-mark]

It would be interesting to use a jsonpath as the basis for a lookup into another object/array.

For example, given the json:

{
  "object": {
    "primaryKey": "foo",
    "foo": "bar",
    "other": "value"
  },
  "array": [
    { "foo": "one" },
    { "foo": "two" }
  ]
  "prefs" { "fieldname": "other" },
  "objIndex": 1
}

It might be handy to be able to:

• $.object[ $.prefs.fieldname ] -> `"value"
• $.object[ @.primaryKey ] -> "bar"
• $.array[ $.objIndex ] -> { "foo": "two" }