Ability not to rebuild configs in case they're up to date

[samdark]

We recently did an extra check in Yii demo: https://github.com/yiisoft/yii-demo/blob/master/public/index.php#L33

It is checking if mtime of any source file is higher than mtime of target, config is rebuilt. Else rebuild stage is ignored. Would be great to adopt it in the config plugin itself and remove from apps index.php.