Allow JSONAPI unsuffixed associations keys

This PR allows default associations keys in JSON-API format, without `_id` or `_ids` suffix, as shown here: https://github.com/json-api/json-api/blob/gh-pages/format/index.md#resource-relationships-

By default, the suffix keeps being added but by setting the `default_key_type` config option to `:name`, singular or plural class names will be used instead: 

By default:
```javascript
//...
  {
    "id": "1",
    "title": "Rails is Omakase",
    "links": {
      "author_id": "9",
      "comment_ids": [ "5", "12", "17", "20" ]
    }
  }
//...
```

And by adding `config/initializers/active_model_serializers.rb`:
`````ruby
ActiveModel::Serializer.setup do |config|
  config.default_key_type = :name
end
````
we get this:
```javascript
//...
  {
    "id": "1",
    "title": "Rails is Omakase",
    "links": {
      "author": "9",
      "comments": [ "5", "12", "17", "20" ]
    }
  }
//...
```

This avoids setting `:key` option on each serializer association when (trying to...) following JSONAPI specs.

Tell me if you aggree this commit. I'll write tests and doc if you do.