r.colors.out: added json support

[NishantBansal2003]

fixes: #3537 Use parson to add json output format support to the r.colors.out and r3.colors.out module. The JSON output looks like as follows:

[
    {
        "value": 1,
        "rgb": "rgb(21, 5, 57)"
    },
    {
        "value": 2,
        "rgb": "rgb(143, 15, 140)"
    },
    {
        "value": 3,
        "rgb": "rgb(95, 225, 24)"
    },
    {
        "value": 4,
        "rgb": "rgb(18, 116, 26)"
    },
    {
        "value": 5,
        "rgb": "rgb(136, 220, 166)"
    },
    {
        "value": 6,
        "rgb": "rgb(250, 245, 99)"
    },
    {
        "value": 7,
        "rgb": "rgb(221, 176, 246)"
    },
    {
        "value": 8,
        "rgb": "rgb(236, 78, 1)"
    },
    {
        "value": 9,
        "rgb": "rgb(2, 197, 45)"
    },
    {
        "value": 10,
        "rgb": "rgb(17, 118, 29)"
    },
    {
        "value": 11,
        "rgb": "rgb(127, 210, 181)"
    },
    {
        "value": 12,
        "rgb": "rgb(129, 13, 6)"
    },
    {
        "value": 13,
        "rgb": "rgb(105, 239, 116)"
    },
    {
        "value": 14,
        "rgb": "rgb(242, 232, 152)"
    },
    {
        "value": 15,
        "rgb": "rgb(90, 161, 179)"
    },
    {
        "value": 16,
        "rgb": "rgb(55, 187, 209)"
    },
    {
        "value": 17,
        "rgb": "rgb(24, 9, 97)"
    },
    {
        "value": 18,
        "rgb": "rgb(127, 247, 104)"
    },
    {
        "value": 19,
        "rgb": "rgb(114, 10, 161)"
    },
    {
        "value": 20,
        "rgb": "rgb(8, 35, 6)"
    },
    {
        "value": 21,
        "rgb": "rgb(220, 204, 139)"
    },
    {
        "value": 22,
        "rgb": "rgb(160, 3, 70)"
    },
    {
        "value": 23,
        "rgb": "rgb(146, 169, 74)"
    },
    {
        "value": 24,
        "rgb": "rgb(202, 8, 65)"
    },
    {
        "value": 25,
        "rgb": "rgb(32, 45, 210)"
    },
    {
        "value": 26,
        "rgb": "rgb(33, 173, 31)"
    },
    {
        "value": 27,
        "rgb": "rgb(62, 132, 123)"
    },
    {
        "value": 28,
        "rgb": "rgb(34, 120, 179)"
    },
    {
        "value": 29,
        "rgb": "rgb(48, 38, 22)"
    },
    {
        "value": "nv",
        "rgb": "rgb(255, 255, 255)"
    },
    {
        "value": "default",
        "rgb": "rgb(255, 255, 255)"
    }
]