[output/i3bar] Add flush method

flush() terminates a single iteration of widget drawing.

see #23

bumblebee/output.py

@@ -29,4 +29,9 @@ class I3BarOutput(object):
             })
         sys.stdout.write(json.dumps(result))
 
+    def flush(self):
+        """Flushes output"""
+        sys.stdout.write(",\n")
+        sys.stdout.flush()
+
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