native-lib-loader option instead of using narSystem.

When we have <narSystemPackage>native-lib-loader</narSystemPackage>, we copy the output to /META-INF/lib according to native-lib-loader convention.

The minimal project for testing it is https://github.com/hkmoon/nar-native-lib.

I would rephrase this to: 

"How do I use the NAR plugin to create a JNI type library for multiple platforms (Windows, MacOSX and Linux)?"