[core/themes] Add "cycling" support

Allow a theme to define a "cycle" of attributes that are cycled through
on a widget-per-widget basis (e.g. for alternating the widget
background). These cycles take precedence over the default values, but
can be overridden by module-specific theme instructions.

see #23

bumblebee/output.py

@@ -57,6 +57,7 @@ class I3BarOutput(object):
     def begin(self):
         """Start one output iteration"""
         self._widgets = []
+        self._theme.reset()
 
     def flush(self):
         """Flushes output"""