Add a JSON output mode

Implement the --output=[plain|json] flag to control whether output should be the legacy text one or structured JSON for further elaboration.

Sample output:

$ gitlab-rulez --output json diff --filter pkg/z* rulez.yaml | jq
{
  "actions": [
    {
      "project": {
        "id": 6315,
        "path_with_namespace": "pkg/zeitgeist"
      },
      "kind": "settings",
      "description": "set default_branch to apertis/v2024dev3"
    },
    {
      "project": {
        "id": 6315,
        "path_with_namespace": "pkg/zeitgeist"
      },
      "kind": "settings",
      "description": "save changes",
      "error": "400: {'base': [\"Could not change HEAD: branch 'apertis/v2024dev3' does not exist\"]}"
    },
    {
      "project": {
        "id": 8265,
        "path_with_namespace": "pkg/zict"
      },
      "kind": "settings",
      "description": "set default_branch to apertis/v2024dev3"
    },
    {
      "project": {
        "id": 8265,
        "path_with_namespace": "pkg/zict"
      },
      "kind": "settings",
      "description": "save changes",
      "error": "400: {'base': [\"Could not change HEAD: branch 'apertis/v2024dev3' does not exist\"]}"
    }
  ]
}

$ gitlab-rulez --output json list-projects --filter pkg/zc* | jq
[
  {
    "id": 10147,
    "path_with_namespace": "pkg/zc.buildout"
  },
  {
    "id": 10148,
    "path_with_namespace": "pkg/zc.lockfile"
  }
]