Render json output

[gabotechs]

As requested in #6, this PR introduces the ability to output a machine readable output in json format with the --json flag:

dep-tree render src/file.ts --json

The output format looks like this:

{
  "tree": {
    "index.ts": {
      "file-1.ts": {
        "file-2.ts": {
          "file-3.ts": null,
          "file-4.ts": null
        },
        "file-4.ts": null
      },
      "file-2.ts": {
        "file-3.ts": null,
        "file-4.ts": null
      },
      "file-3.ts": null
    }
  },
  "circularDependencies": [],
  "errors": {
    "file-1.ts": [
      "could not import file-4275.ts, not found"
    ],
    "file-3.ts": [
      "could not import file-1423.ts, not found"
    ]
  }
}

If a node is a leaf node (meaning that it is not importing more other files), its value will be null, otherwise, its value will be a key-value object that goes from its file path to either another key-value object or null.

circularDependencies is of type [][]String, and it will look something like this:

{
"circularDependencies": [
    [
      "file-2.ts",
      "file-3.ts",
      "file-4.ts",
      "file-2.ts"
    ]
  ]
}

closing the loop of the circular import stack.

While parsing files, it can happen that there are some errors. Instead of failing, dep-tree just stores the errors and keeps trying to parse the file. Those errors will be gathered in the errors key in the json output, which is a map from file to array of errors in that file.

[belong-bryce]

This looks great!