add namespace support for translated string

[bruceman]

I think namespace is very important for large project to isolate the different i18n resource.
Think about the same string with different translated string on different context.
This pull just is a referenced and fast implement, without block any exist test or use cases.
please help integrate this feature if possible.

Thanks.

[rubenv]

Think about the same string with different translated string on different context.

We have a concept for this: contexts.

This is basically the same as gettext domains, which we won't add, please refer to e.g. #115 for background.