Add plural_default_root config option

This PR adds a `plural_default_root` config options which will take the pluralized name by default as root key to match JSON-API specs (https://github.com/json-api/json-api/blob/gh-pages/format/index.md#individual-resource-representations-)

The default behavior keeps using singular root name for individual resources, but by setting `config.plural_default_root = true` in an initializer, all serializer will take pluralized versions by default, avoiding to set it in each serializer.