Add subLocale to easy_localization

[enricostrijks]

This way you could provide multi language for people. Say for example a learning app like Duolingo, hereby you can then specify the main and sub language.

Or if you want to have a mix of two dialects you can do it this way.

[bw-flagship]

To be honest, I am not convinced that this should be merged into easy_localization. Having two locales is a very niche feature that is not relevant for the vast majority of apps.
At the same time, it increases the complexity a lot and all future developers need to maintain this functionality.

I would suggest to keep this in a fork, but not merge it to the main project.

Open for different opinions.