[themes] Add support for generic warning/critical colors

Font and background colors for warning and critical elements can now be
specified using fg-warning, fg-critical, bg-warning and bg-critical.

Also, optionally, the "urgent" flag will be set towards the i3bar, if
possible.

bumblebee/module.py

@@ -6,6 +6,12 @@ class Module(object):
     def data(self):
         pass
 
+    def critical(self):
+        return False
+
+    def warning(self):
+        return False
+
     def state(self):
         return "default"