Option to generate files with untranslated strings only

Added an option to generate Apple string files only with the strings that are not translated.

The typical use case for this is incremental translation of an existing app.
This will extract only the strings that are not translated, and the generated file can be sent to a translator or an online translation service.

It's specially useful for online translation services where you can upload directly the Localizable.strings file. 

This way you don't need to find or extract manually the strings that need to be translated.