Revamp build process to separate locale data

This changes the build process of this package to separate the locale data from the runtime lib.

When using this package in Node.js, the library will still be primed with all its locale data. _But_ on the client, it won't. The `dist/` dir now has the following structure:

```
dist/
├── intl-messageformat-with-locales.js
├── intl-messageformat-with-locales.min.js
├── intl-messageformat.js
├── intl-messageformat.js.map
├── intl-messageformat.min.js
└── locale-data
    ...
    ├── en.js
    ...
    ├── fr.js
    ...
    └── yo.js

1 directory, 157 files
```

----

After much thought and discussion, this also _always_ defaults `IntlMessageFormat` to English (although a developer can override this), and the English locale data is the only data included by default in the browser.

At first there was a change to automatically detect the default locale, but I realized that this won't work on the server because it would be nonsensical for any given request.