[output] Add support for pango markup

Add a new parameter "output.markup" that allows a user to pass in a
custom markup string (e.g. "pango").

Note: To make use of this, the user still has to use a Pango font, as
well as use a bumblebee-status module that supports Pango output.

fixes #493

bumblebee/output.py

@@ -164,6 +164,7 @@ class I3BarOutput(object):
             "align": self._theme.align(widget),
             "instance": widget.id,
             "name": module.id,
+            "markup": self._config.get("output.markup", "none"),
         })
 
     def begin(self):