Simpler options

Greatly simplified the configuration process using user-specified options:

- 2 new options:
  - `defaultLocale` allows users to specify the default/fallback locale when none was specified or could be derived
  - `messages` allows users to specify a pre-defined message bundle and avoid fetching
- Options are no longer validated as we should trust the user to be sensible and save precious load time
- *Private* functions were moved to their own shared utility namespace: `_`
- Improve modularity
- Core classes (i.e. `Internationalization`, `Messenger`) are now exposed