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/Jamfile

@@ -0,0 +1,19 @@
+#
+# Copyright (c) 2016-2017 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/boostorg/beast
+#
+
+exe websocket-chat-multi :
+    http_session.cpp
+    listener.cpp
+    main.cpp
+    shared_state.cpp
+    websocket_session.cpp
+    :
+    <variant>coverage:<build>no
+    <variant>ubasan:<build>no
+    ;