If no configuration file is present do not crash.

If no configuration file was found, the direct access to the key would
raise an exception, not catched. Using the get method fixes this.

cmus_notify/cmus_notify.py

@@ -22,7 +22,7 @@ def load_notifier(arguments):
     :param arguments: The parsed arguments
     :returns: The notifier to be used for the notifications
     """
-    custom_notifier_path = arguments['custom_notification']
+    custom_notifier_path = arguments.get('custom_notification', None)
     if custom_notifier_path:
         sys.path.insert(0, os.path.expanduser(os.path.dirname(custom_notifier_path)))
         notifier_module = importlib.import_module(os.path.basename(custom_notifier_path))