[core/theme] Add a way to exclude themeing for widget

Allow a widget to set "theme.exclude" to extend any part of a theme
(e.g. the suffix). Note that this can later, on demand, be extended to
allow a list of elements to be passed in.

see #401

bumblebee/theme.py

@@ -223,6 +223,9 @@ class Theme(object):
         if not self._widget:
             self._widget = widget
 
+        if self._widget.get("theme.exclude", "") == name:
+            return None
+
         if self._widget != widget:
             self._prevbg = self.bg(self._widget)
             self._widget = widget