GoIgniter, Extending CodeIgniter 3.

@Diederik : Excellent insight !!! GoIgniter has some kind of multisite capability. Thus, I should "whitelist" allowed hostname before autoguessing based on $_SERVER['SERVER_NAME']

@ivantcholakov : The reason I put "twig" in core directory is because I want to let people use twig without installing composer. Probably the best way is firstly check whether composer's autoloader is exists. If it exists, then I should use composer's autoloader, otherwise, I can fallback to this old way. Thanks for your insight.