Add a --pretty option to format JSON output for human consumption

Without --pretty

`{"root": {"customer": [{"last": "Baker", "first": "Scott"}, {"last": "Doolis", "first": "Jason"}]}}`

**With** --pretty
~~~
{
    "root": {
        "customer": [
            {
                "first": "Scott",
                "last": "Baker"
            },
            {
                "first": "Jason",
                "last": "Doolis"
            }
        ]
    }
}
~~~