[feature] default language keys when missing in a split_export_files

Hi and thanks a lot for this great and useful package.

It could be nice to have an option that make generation of split_export_files merging default language keys when they are missing in a language file.

For example with those languages keys in resources/langs :
- en [default]
- foo
- bar
- fr
- foo

we could navigate in language "fr" and displaying the "en [default]" value in place of the key path.

Also it make Javascript translation works like the Php Laravel side with a default language value when a language missing a key.

I Hope to be clear enough ... I'm not sure :-)