Add default language to Intl

Hi,

First, thank you for this service. I'm using it only for Intl polyfill, as Safari does not have it by default. Polyfills.io correctly detects lack of Intl support in Safari and returns the Intl polyfill, however this library requires a language to be loaded too.

This PR just adds "english" as a default language, so that the library at least works out of the box.

I think a better way to solve this issue would be to allow to specify to Polyfills which locale you are interested in, something like "+intl.en,intl.fr", and it would return the Intl AND also all the locales provided.

All locales data can be found here: https://github.com/andyearnshaw/Intl.js/tree/master/locale-data/jsonp