[modules] Add NIC module

Add a module that displays the status of all NICs (interface name, list
of IPs and state).

In its status, it also exposes whether it's a WiFi or a wired NIC.

For this functionality, additional code was implemented to allow a
module to add multiple elements to the bar at once. The framework calls
the module until its "next()" method return False.

bumblebee/module.py

@@ -15,4 +15,7 @@ class Module(object):
     def state(self):
         return "default"
 
+    def next(self):
+        return False
+
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