Provide private API to pass application class loader for nashorn script engine

Nashorn script engine uses caller class loader as script "application loader" when security manager is present. If no security manager is present, System class loader is used. It is better if the default application loader is thread context class loader (when no security manager is present). Also it is better if NashornScriptEngineFactory class non-standard/private API to pass a specific classloader as script application loader.