Add native jar mode

[philwebb]

Running native-image on a jar is a bit painful:

$ rm -rf target/native
$ mkdir -p target/native
$ cd target/native
$ jar -xvf ../myproject-0.0.1-SNAPSHOT.jar
$ native-image -H:Name=myproject @META-INF/native-image/argfile -cp .:BOOT-INF/classes:`find BOOT-INF/lib | tr '\n' ':'`
$ mv my-app ../

We could include a jartool to take care of this similar to the LayerToolsJarMode.