Enable explicit instantiation of websocket::stream:

fix #1279, close #1319

This enables users to improve compilation performance by explicitly
instantiating the stream template in another TU.

Signed-off-by: Damian Jarek <damian.jarek93@gmail.com>

test/beast/websocket/stream_explicit.cpp

@@ -0,0 +1,15 @@
+//
+// Copyright (w) 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
+//
+
+// Test that header file is self-contained.
+#include <boost/beast/websocket/stream.hpp>
+#include <boost/asio/ip/tcp.hpp>
+
+template class boost::beast::websocket::stream<boost::asio::ip::tcp::socket, false>;
+template class boost::beast::websocket::stream<boost::asio::ip::tcp::socket, true>;