[Feat] Add JSON output to r.colors.out

[petrasovaa]

Add JSON output for easy parsing of r.colors.out output.

Describe the solution you'd like
Adding option format, it would be something like format=plain for the current output format and format=json for the JSON output. There is a newly added parson library to GRASS (#3028), so it should be used here.

See also discussion in #3019.

A test should be included.
A possible JSON layout could be:

[
  {
    "value": 0,
    "red": 255,
    "green": 0,
    "blue": 0
  },
  {
    "value": 1,
    "red": 0,
    "green": 255,
    "blue": 0
  },
  {
    "value": 2,
    "red": 0,
    "green": 0,
    "blue": 255
  }
]

[cwhite911]

It might be nice to have the output in a usable formats.

[
 {
    "value": 0,
    "hex": "#FF0000",
    "rgb": "rgb(255, 0, 0)",
    "rgb": "rgb(255, 0, 0, 0)",
    "hsl":  "hsl(0, 100, 50)"
  }
]