Support a row key for JSON data

[davidmegginson]

Some JSON data might look like this:

[
    {
        "attributes": {
            "#org": "UNICEF",
            "#sector": "Education"
        },
        "metadata:" {
            "foo": "bar"
        }
    },
    {
        "attributes": {
            "#org": "UNHCR",
            "#sector": "Shelter"
        },
        "metadata:" {
            "foo": "bar"
        }
    }
]

We need a way to extract the data row from each JSON object in the list. This is different than the top-level data selector.

[davidmegginson]

Fixed via #248 by implementing an advanced JSONPath parser.