Support external refs

[mre]

It would be great if schemafy could support external refs like these:

{
  "$schema": "http://json-schema.org/draft-04/schema#",
  "type": "object",
  "properties": {
    "reactions": {
      "type": "array",
      "items": { "$ref": "reaction.json" }
    }
  },
  "id": "file_comment"
}

(As discussed in #2)