echo_server now checks for a special command to stop listening for new connections

examples/echo_server/echo_server.cpp

@@ -19,6 +19,13 @@ void on_message(server* s, websocketpp::connection_hdl hdl, message_ptr msg) {
               << " and message: " << msg->get_payload()
               << std::endl;
 
+    // check for a special command to instruct the server to stop listening so
+    // it can be cleanly exited.
+    if (msg->get_payload() == "stop-listening") {
+        s->stop_listening();
+        return;
+    }
+
     try {
         s->send(hdl, msg->get_payload(), msg->get_opcode());
     } catch (const websocketpp::lib::error_code& e) {