r.profile: add JSON support

[kritibirda]

Use parson to add json output format support to the r.profile module.

The output format is as follows:

[
    {
        "easting": <float>,
        "northing": <float>,
        "distance": <float>,
        "elevation": <float>,
        "red": <int>,
        "green": <int>,
        "blue": <int>
    },
    {
        "easting": <float>,
        "northing": <float>,
        "distance": <float>,
        "elevation": <float>,
        "red": <int>,
        "green": <int>,
        "blue": <int>
    },
...
]

Note that the elevation field is the name of the input and red, green, blue fields are only included if the colors flag is specified.