Allow config items to be present in any of the configuration files, not just in config/session.php

Without this patch $this->app_name would end up as empty string if $config['app_name'] isn't defined inside config/session.php. This happens because Codeigniter wouldn't pass $config['app_name'] to constructor if it's defined elsewhere like inside config/config.php. Using config/config.php or other configuration file is sometimes required because 'app_name' config item has the potential to be used for namespacing purposes by other code inside an average application.