Allow subclassing Java classes from script without creating instances

Currently, the only way to subclass a Java class in Nashorn is to create an instance a la anonymous inner classes in Java:

    var r = new java.lang.Runnable() {
        run: function() { ... }
    }

However, sometimes we need to create a subclass without creating instances. We'd need a functionality like:

    var R = Java.extend(java.lang.Runnable, {
        run: function() { ... }
    })

Such a class can then be instantiated:

   var r = new R

Or passed to some other API, i.e.:

  launch(R.class)