Refactor for IntlRelativeFormat and CLDR data

This is a fairly major refactor of this package to bring it inline with `react-intl`. The changes include the following:

- Add `{{formatRelative}}` helper which uses `IntlRelativeFormat`.
- Include all CLDR data for the 150+ supported locales.
- Revamp build process to include locale data in `dist/`.

Since the locale data has been separated from the runtime, to use this package, a developer must now include two scripts on the page; e.g., for French:

```html
<script src="handlebars-intl/handlebars-intl.min.js"></script>
<script src="handlebars-intl/locale-data/fr.js"></script>
```

Making these changes brings down the total size of this library — when used in the browser, bundled with its dependencies — to **7.1k gz**.