Add support for filtering included items #137

The JSON API spec suggests that an API may support an include parameter when fetching resources that allows the caller to specify which related resources to add to the included array. The current version of this library either marshalls all resources or none, depending on which API is used. The returned structs (OnePayload, ManyPayload) do not expose the underlying Included array of Nodes that contain the included items or any other way to control what is serialized. It would be great if the API can either add support for passing in which Node.Types to add to this array or at least expose it to the returned Payloader interface so callers can filter it themselves.