Support nested JSON locale data

This release supports nested JSON locale data. Here is example:

const locales: ILocale[] = [
  {
    language: 'en',
    messages: {
      page1: {
        hello: 'Hello world!',
      },
      page2: {
        hello: 'Hello eo-locale!',
      },
    },
  },
];

const translator = new Translator('en', locales);
translator.translate('page1.hello'); // Hello world!