Removing invalid plural keys when using add-missing

[ChrisLabattD2L]

This isn't a defect, as much as a suggestion, but it would be beneficial if running add-missing would remove invalid plural keys for localizations that were making use of the plural format.

For example:

In English "{number, plural, one {# thing} other {# things}}" would become "{number, plural, other {# things}}" in Japanese, Korean, etc., since one doesn't work in those languages.