This commit adds a global `--json` flag that structures the output of Rover like so:

**success:**

{
  "data": {
    "sdl": {
      "contents": "type Person {\n  id: ID!\n  name: String\n  appearedIn: [Film]\n  directed: [Film]\n}\n\ntype Film {\n  id: ID!\n  title: String\n  actors: [Person]\n  director: Person\n}\n\ntype Query {\n  person(id: ID!): Person\n  people: [Person]\n  film(id: ID!): Film!\n  films: [Film]\n}\n",
      "type": "graph"
    }
  },
  "error": null
}

**errors:**

{
  "data": null,
  "error": {
    "message": "Could not find subgraph \"products\".",
    "suggestion": "Try running this command with one of the following valid subgraphs: [people, films]",
    "code": "E009"
  }
}