Add relative formatting support and locale data

This refactor brings `dust-intl` inline with the other integration packages: `handlebars-intl` and `react-intl`. This package now supports relative-time formatting and also includes all the locale data for the 150+ supported locales (English is baked in as the default.)

_Note: The changes to `dist/` are left out of this PR so it's reviewable._
trailing comma!