Add support for a top-level "linked" section

According to the JSON-API spec, associated objects can be included in a top level "linked" section, e.g. (from the spec)

```json
{
  "posts": [{
    "id": "1",
    "title": "Rails is Omakase",
    "links": {
      "author": "9"
    }
  }],
  "linked": {
    "people": [{
      "id": "9",
      "name": "@d2h"
    }]
  }
}
```

This PR adds support for that.