Allow passing routing options to jsonapi_relationships

[biomancer]

This allows passing routing options to jsonapi_relationships, which is useful if you are building read-only API, and want to limit generated methods to GET-only:

with_options(only: [:index, :show]) do
    jsonapi_resources :contacts do
      jsonapi_relationships
    end
end

jsonapi_links and jsonapi_related_resources already support these options, so it seemed logical to add it here too.

[lgebhardt]

Thanks!