Add a way not to include all locales

There should be a means of not including all locales.

A very simple solution would be to move the logic to `lib/core` and keep `lib/index` including all locales, so people would be able to manually require just the logic and the locales they'll use:
```javascript
import writtenNumber from 'written-number/lib/core';
import writtenNumberEn from 'written-number/lib/i18n/en';
// ...
```