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/beast.hpp

@@ -0,0 +1,19 @@
+//
+// 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_BEAST_HPP
+#define BOOST_BEAST_EXAMPLE_WEBSOCKET_CHAT_MULTI_BEAST_HPP
+
+#include <boost/beast.hpp>
+
+namespace beast = boost::beast;                 // from <boost/beast.hpp>
+namespace http = beast::http;                   // from <boost/beast/http.hpp>
+namespace websocket = beast::websocket;         // from <boost/beast/websocket.hpp>
+
+#endif