api structure (MVC)
  - api
     - users
        - index.js
     - index.js
  - controllers
     - userController.js
     - userVocabularyController.js
     - vocabularyController.js
     - index.js
  - models
     - index.js
     - users
          - user.js
     - userVocabularies
         - userVocabulary.js
     - vocabularies
         - vocabulary.js