[output] Allow modules to pass back a single widget

Since "single widget" is the 99% use-case, make it easier for a module
to return a single widget.

bumblebee/output.py

@@ -66,6 +66,11 @@ class Output(object):
         pass
 
     def draw(self, widgets, theme):
+        if not type(widgets) is list:
+            widgets = [ widgets ]
+        self._draw(widgets, theme)
+
+    def _draw(self, widgets, theme):
         pass
 
     def flush(self):