cookie_prefix fix and a few extra functions for reloading language files

get_lang_by_cookie function didn't take into account CI's cookie_prefix as CI Input library doesn't check it WHEN READING COOKIES.

Also added reset_languages() and reload_languages() functions. These are useful when changing languages on the fly and you want to reload language files. They don't support HMVC (loading language files inside a module) unfortunately, maybe someone else can fix that.