Support packages?

Not sure if this is a needed feature, but Atom comes with many builtin features implemented with packages. If `localization` accepts package localization, then this pull request would translate builtin packages' menu items.

If not, I'll try to maintain a branch of package translations at [rschiang/Atom-Localization](/rschiang/Atom-Localization).

Kudos for the translation script though. :smiley: