[core/output] Make bar actually work (flush output)

Seems adding a \n isn't sufficient for flushing, instead explicitly
flush stdout to produce a result reliably in the i3bar.

core/output.py

@@ -19,6 +19,7 @@ class i3(object):
         if 'suffix' in data:
             sys.stdout.write(data['suffix'])
         sys.stdout.write('\n')
+        sys.stdout.flush()
 
     def start(self):
         return {