Added parameter to identify app instance in the source URL

In OSMF.swc, there is a class called StreamingURLResource, its constructor takes the optional argument urlIncludesFMSApplicationInstance. It was not being used, but now it is exposed through this plugin's parameter useAppInstance (default value is false to behave as before):

rtmpConfig: {
  swfPath: 'dist/assets/RTMP.swf',
  useAppInstance: true
}