add example to shutdown the proxy master

examples/addons/shutdown.py

@@ -0,0 +1,18 @@
+"""
+A simple way of shutting down the mitmproxy instance to stop everything.
+
+Usage:
+
+    mitmproxy -s shutdown.py
+
+    and then send a HTTP request to trigger the shutdown:
+    curl --proxy localhost:8080 http://example.com/path
+"""
+from mitmproxy import ctx, http
+
+
+def request(flow: http.HTTPFlow) -> None:
+    # a random condition to make this example a bit more interactive
+    if flow.request.pretty_url == "http://example.com/path":
+        ctx.log.info("Shutting down everything...")
+        ctx.master.shutdown()