allow skipping building debug builds in vcpkg tree

[mcgoo]

No description provided.

[waych]

This is now possible using a bit of configuration on the part of the crate using cargo-vcpkg. By specifying ones own triplet files for vcpkg, one can define and use triplets that only include either release, or debug builds as part of the triplet build. See #4 for an example.

Seeing as this sort of configuration constricts upstream builds to exclusively debug or release, it seems inappropriate to try to support the functionality directly in this crate (for loss of generality using the overlay triplet solution).