Add option for json formatted for rickshaw.

Rickshaw graphs expect data in a certain format, and
having that format as an output option in graphite
simplifies the use of rickshaw graphs.

https://github.com/shutterstock/rickshaw
``` json
[
    {
        "color": "blue",
        "name": "New York",
        "data": [ { "x": 0, "y": 40 }, { "x": 1, "y": 49 }, { "x": 2, "y": 38 }, { "x": 3, "y": 30 }, { "x": 4, "y": 32 } ]
    }, {
        "name": "London",
        "data": [ { "x": 0, "y": 19 }, { "x": 1, "y": 22 }, { "x": 2, "y": 29 }, { "x": 3, "y": 20 }, { "x": 4, "y": 14 } ]
    }, {
        "name": "Tokyo",
        "data": [ { "x": 0, "y": 8 }, { "x": 1, "y": 12 }, { "x": 2, "y": 15 }, { "x": 3, "y": 11 }, { "x": 4, "y": 10 } ]
    }
]
```