[core/output] Add generic pango support

Allow any piece of a theme that specifies a set of attributes (default,
cycles, states, widgets) to use pango *instead* of the usual attributes.

If pango is present, this will have precedence.

A practical example of this can be found in the powerline-pango theme,
which is added solely for demonstration purposes.

fixes #531

core/theme.py

@@ -44,6 +44,7 @@ class Theme(object):
             ('border-right', 0),
             ('padding', ''),
             ('prefix', ''), ('suffix', ''),
+            ('pango', None),
         ]:
             setattr(self, attr.replace('-', '_'), lambda widget=None, default=default, attr=attr: self.__get(widget, attr, default))