Addition of `afterPublish` callback

Addition of ```afterPublish``` callback, Will be called after all files and directories have been copied.

I needed this to simplify using CKEditor via bower with some custom plugins.
After CKEditor's files have been copied my custom plugins will be copied too.

e.g.
```
class CKEditorAsset extends \yii\web\AssetBundle
{
    public $sourcePath = '@vendor/bower/ckeditor';
    public $sourcePathExtra = '@common/widgets/ckeditor';
    ...
    public function init()
    {
        parent::init();
        $this->publishOptions['afterPublish'] = [$this, 'afterPublish'];
    }

    public function afterPublish($am, $dst)
    {
        \yii\helpers\FileHelper::copyDirectory(\Yii::getAlias($this->sourcePathExtra), $dst, [
            'dirMode' => $am->dirMode,
            'fileMode' => $am->fileMode,
        ]);
    }
}
```