game: allow overriding the config directory location (#3477)

This is primarily driven for proper mod-support. Mods would like to
isolate their settings and saves (potentially) and that is currently
done by find-and-replacing code before building. Bad!

Additionally, this has the side-effect of allowing for portable
installations of the game so, win-win.

Testing in progress, i'll merge once it is ready.

game/system/hid/input_manager.cpp

@@ -22,6 +22,7 @@ InputManager::InputManager()
   prof().instant_event("ROOT");
   {
     auto p = scoped_prof("input_manager::init");
+    m_settings->load_settings();
     {
       auto p = scoped_prof("input_manager::init::sdl_init_subsystem");
       // initializing the controllers on startup can sometimes take a very long time