add example how to replace response body contents

examples/modify_response_body.py

@@ -0,0 +1,18 @@
+# Usage: mitmdump -s "modify_response_body.py mitmproxy bananas"
+# (works best with --anticache)
+
+import sys
+from libmproxy.protocol.http import decoded
+
+
+def start(ctx, argv):
+    if len(argv) != 3:
+        ctx.log('Usage: -s "modify-response-body.py old new"')
+        sys.exit(1)
+    global old, new  # In larger scripts, a centralized options object (as returned by argparse) is encouraged
+    old, new = argv[1:]
+
+
+def response(ctx, flow):
+    with decoded(flow.response):  # automatically decode gzipped responses.
+        flow.response.content = flow.response.content.replace(old, new)