Likun/validate translation json

@zendesk/quokka 

:koala: :+1: 

This PR deals with validation on en.json for zendesk i18n format. When `package` is defined inside `app` element, then all the elements should have `title` and `value` defined. 

It also adds more options to `to_flattened_namespaced_hash` method to allow us flatten the JSON either for i18n or not.