r.category: add JSON support

[kritibirda26]

The JSON output looks as follows:

[
    {
        "category": 1,
        "description": "trees, very green"
    },
    {
        "category": 2,
        "description": "water, very deep"
    }
]

Note that the format option for this module is named as output_format because format is being used as the name for another option.

[cwhite911]

@kritibirda26 overall this looks good.

I have a few thoughts about some additional features we could add, but they are outside of the scope of this PR. So, just for a quick discussion should we make some new feature issues for the following ideas.

1. Color option - We should add a new option that outputs the category color. In any use case where the user wants to create a visualization they will want to know the category label and color.
2. JSON input - Allow user to use json as the rules file. As part of this we might just want to look at a way to allow this for all spots where we have a text rules files.

[kritibirda26]

Hi @cwhite911 and @echoix, I have updated the PR according to the feedback and created issues for the suggested new features. The PR should be ready to merge.

[echoix]

I have nothing to say against, but I would prefer another approval to be confident.