save reference to the labeltool in the command

sloth/core/cli.py

@@ -97,7 +97,7 @@ class BaseCommand(object):
 
     def __init__(self):
         #self.style = color_style()
-        pass
+        self.labeltool = None
 
     def usage(self, subcommand):
         """

sloth/core/labeltool.py

@@ -161,6 +161,10 @@ class LabelTool(QObject):
         else:
             # TODO implement load_command_class
             klass = load_command_class(app_name, subcommand)
+
+        # set labeltool reference
+        klass.labeltool = self
+
         return klass
 
     def init_from_config(self, config_module_path=""):