[core/theme] Add preliminary cycling support

Add support for cycling values in a theme, for now as long as the cycle
size is 0 :P

core/output.py

@@ -43,6 +43,7 @@ class i3(object):
                 'name': module.id(),
                 'color': self._theme.fg(widget),
                 'background': self._theme.bg(widget),
+                'separator': self._theme.default_separators(),
             })
         return widgets