add websocket example

examples/simple/websocket_messages.py

@@ -0,0 +1,13 @@
+import re
+from mitmproxy import ctx
+
+
+def websocket_message(flow):
+    # get the latest message
+    message = flow.messages[-1]
+
+    # simply print the content of the message
+    ctx.log.info(message.content)
+
+    # manipulate the message content
+    message.content = re.sub(r'^Hello', 'HAPPY', message.content)