Support for KEEP-45 - add a possibility to explicitly add public modifier

[Schahen]

In consistency with https://github.com/Kotlin/KEEP/issues/45

I still believe that we'd rather have an annotation for suppressing this but we gotta start with something.

[Schahen]

Also it makes sense to support internal attribute.

[Colerar]

How is it going now? I'm very happy if dukat can support the explicit api mode.