Implement NetworkManager

For now there are two methods:

- `NETWORK:IsUrlAllowed()`: Check whether access to a certain URL is allowed.
- `NETWORK:HttpRequest()`: Perform an HTTP request.

By default access to the network is disabled for all target hosts. It
can be enabled by setting `HttpEnable=1` in the preferences. Individual
hosts have to be added to `HttpAllowHosts` as a comma separated list to
allow access.

See included docs for more details on usage.

extern/IXWebSocket-11.3.2/test/test_runner.cpp

@@ -0,0 +1,29 @@
+/*
+ *  test_runner.cpp
+ *  Author: Benjamin Sergeant
+ *  Copyright (c) 2018 Machine Zone. All rights reserved.
+ */
+
+#define CATCH_CONFIG_RUNNER
+#include "catch.hpp"
+#include <ixwebsocket/IXNetSystem.h>
+#include <spdlog/spdlog.h>
+
+#ifndef _WIN32
+#include <signal.h>
+#endif
+
+int main(int argc, char* argv[])
+{
+    ix::initNetSystem();
+
+#ifndef _WIN32
+    signal(SIGPIPE, SIG_IGN);
+#endif
+    spdlog::set_level(spdlog::level::debug);
+
+    int result = Catch::Session().run(argc, argv);
+
+    ix::uninitNetSystem();
+    return result;
+}