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

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.