Automatically pull requested known locales

A little workarround to check first if the locale is not already known by messageformat, before throwing any "Plural Function not found" error.
Would be a better improvement to "automatically pull in all known locales" as described in README.md?
Fix client-side previously introduced bug

module AND module.exports are not know by the browser (by default)
Merge pull request #21 from jokesterfr/add_known_locales

Add known locales