add data option on relationships

[danivek]

Add a data option function that returns the data to serialize for the relationship.

Example:
Input data can contains meta information ( e.g. total) on data for a relationship.

{
  "id": "1",
  "title": "article 1",
  "comments": {
    "total": 2,
    "data": [
      {
        "id": "1",
        "title": "comment 1"
      },
      {
        "id": "2",
        "title": "comment 2"
      }
    ]
  }
}

Add a data option as a function to return data to serialize for the relationship:

const Serializer = new JSONAPISerializer();

Serializer.register('article', {
  relationships: {
    comments: {
      type: 'comment',
      data: (input) => input.data
    }
  }
});
Serializer.register('comment');

[coveralls]

Coverage remained the same at 100.0% when pulling cc9e83d on add-data-options-for-relationships into b016d08 on master.