Consider allowing a licence arg/option to gen_build_npm so output can live in third_party

[mattnathan]

I'm not that experienced with bazel but I feel like it would be fitting for the generated BUILD/shrikwrap files to exist in the third_party dir of a bazel project.

Unfortunately all BUILD files under third_party require a licence and there's no way to get gen_build_npm to include one in the created BUILD files.