[WIP] SluggableBehavior

Usage:

```php
class Post extends ActiveRecord
{
    public function behaviors() {
        return [
            [
                'class' => Sluggable::className(),
                // use this
                'attribute' => 'title',
                // or this to customize
                // 'value' => function ($event) {
                //     return Inflector::slug($this->title);
                // },
            ],
        ];
    }
}
```