Add websocket-chat-multi example:

This is the multi-threaded io_context version of the
CppCon2018 WebSocket chat server and client example.

example/websocket/server/chat-multi/net.hpp

@@ -0,0 +1,18 @@
+//
+// Copyright (c) 2018 Vinnie Falco (vinnie dot falco at gmail dot com)
+//
+// Distributed under the Boost Software License, Version 1.0. (See accompanying
+// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+//
+// Official repository: https://github.com/vinniefalco/CppCon2018
+//
+
+#ifndef BOOST_BEAST_EXAMPLE_WEBSOCKET_CHAT_MULTI_NET_HPP
+#define BOOST_BEAST_EXAMPLE_WEBSOCKET_CHAT_MULTI_NET_HPP
+
+#include <boost/asio.hpp>
+
+namespace net = boost::asio;                    // from <boost/asio.hpp>
+using tcp = boost::asio::ip::tcp;               // from <boost/asio/ip/tcp.hpp>
+
+#endif