add Application.launch_new_instance classmethod

allows uniformity when making changes.

The change here that prompted it is passing arguments to the instance constructor and initialize.

IPython/config/application.py

@@ -523,6 +523,13 b' class Application(SingletonConfigurable):'
         self.log.debug("Exiting application: %s" % self.name)
         sys.exit(exit_status)
 
+    @classmethod
+    def launch_new_instance(cls, argv=None, **kwargs):
+        """Launch a global instance of this Application"""
+        app = cls.instance(**kwargs)
+        app.initialize(argv)
+        app.start()
+
 #-----------------------------------------------------------------------------
 # utility functions, for convenience
 #-----------------------------------------------------------------------------