[modules] Allow modules to provide default click actions

Pass the "output" object to the modules' constructor to allow them to
define their own callbacks.
Any user-provided callbacks take precedence and override those of the
module.

bumblebee/modules/memory.py

@@ -12,7 +12,7 @@ def description():
     return "Shows available RAM, total amount of RAM and the percentage of available RAM."
 
 class Module(bumblebee.module.Module):
-    def __init__(self, args):
+    def __init__(self, output, args):
         super(Module, self).__init__(args)
         self._mem = psutil.virtual_memory()