[core/module] allow modules to perform updates in background

a module can now set `self.background = True` in its `__init__()` method
to make sure its update method is invoked in a separate thread.

also, do a PoC implementation of this for the github module.

TODO: add this to dev doc

see #640

bumblebee_status/modules/contrib/github.py

@@ -34,6 +34,7 @@ class Module(core.module.Module):
     def __init__(self, config, theme):
         super().__init__(config, theme, core.widget.Widget(self.github))
 
+        self.background = True
         self.__count = 0
         self.__label = ""
         self.__requests = requests.Session()