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/third_party/cpp-linenoise/example/example.cpp

@@ -0,0 +1,54 @@
+#include <iostream>
+#include "../linenoise.hpp"
+
+using namespace std;
+
+int main(int argc, const char** argv)
+{
+    const auto path = "history.txt";
+
+    // Enable the multi-line mode
+    linenoise::SetMultiLine(true);
+
+    // Set max length of the history
+    linenoise::SetHistoryMaxLen(4);
+
+    // Setup completion words every time when a user types
+    linenoise::SetCompletionCallback([](const char* editBuffer, std::vector<std::string>& completions) {
+        if (editBuffer[0] == 'h') {
+#ifdef _WIN32
+            completions.push_back("hello こんにちは");
+            completions.push_back("hello こんにちは there");
+#else
+            completions.push_back("hello");
+            completions.push_back("hello there");
+#endif
+        }
+    });
+
+    // Load history
+    linenoise::LoadHistory(path);
+
+    while (true) {
+        std::string line;
+#ifdef _WIN32
+        auto quit = linenoise::Readline("hello> ", line);
+#else
+        auto quit = linenoise::Readline("\033[32mこんにちは\x1b[0m> ", line);
+#endif
+
+        if (quit) {
+            break;
+        }
+
+        cout <<  "echo: '" << line << "'" << endl;
+
+        // Add line to history
+        linenoise::AddHistory(line.c_str());
+
+        // Save history
+        linenoise::SaveHistory(path);
+    }
+
+    return 0;
+}