Public interface uses private types (non re-exported)

Hi!

I like the simplicity and pretty, slim format :)
I wanted to use a different target, but I noticed, that the `Target` type from env_logger is used, but not re-exported.

Would be great to have all types in the public interface (e.g. `Builder`) re-exported, so that I don't have to add and synchronize another library version.

Thanks!