v.info: add json output for columns

[NishantBansal2003]

Use parson to add json support to v.info for columns. The JSON output looks like as follows:

{
    "columns": [
        {
            "name": "cat",
            "sql_type": "INTEGER",
            "is_number": true
        },
        {
            "name": "MAJORRDS_",
            "sql_type": "DOUBLE PRECISION",
            "is_number": true
        },
        {
            "name": "ROAD_NAME",
            "sql_type": "CHARACTER",
            "is_number": false
        },
        {
            "name": "MULTILANE",
            "sql_type": "CHARACTER",
            "is_number": false
        },
        {
            "name": "PROPYEAR",
            "sql_type": "INTEGER",
            "is_number": true
        },
        {
            "name": "OBJECTID",
            "sql_type": "INTEGER",
            "is_number": true
        },
        {
            "name": "SHAPE_LEN",
            "sql_type": "DOUBLE PRECISION",
            "is_number": true
        }
    ]
}

fixes: #4218

[wenzeslaus]

Looks good and it has a test for most of the functionality. Thanks!

[nilason]

Some minor comments.

[nilason]

As print_columns() creates its own json objects, the following part in main:

grass/vector/v.info/main.c

Lines 57 to 60 in 24469d8

	if (format == JSON) {
	root_value = json_value_init_object();
	root_object = json_value_get_object(root_value);
	}

may be moved down, before:

grass/vector/v.info/main.c

Lines 95 to 96 in 24469d8

	if ((shell_flag & SHELL_BASIC) || format == JSON) {
	print_shell(&Map, field_opt, format, root_object);

[nilason]

Looks good to me now.

[NishantBansal2003]

Could we move forward with this PR?
CC: @cwhite911

Any suggestions are welcome.