Add more conditions to check when unique equals true

Add more conditions to check when unique equals true.

In my case, my "slug" column isnt "unique". But i can't have duplicate slugs for the same "company" (for example). 

Example:
```
protected $sluggable = ['on_slugcheck' => 'addCheckCompanyCondition'];

public function addCheckCompanyCondition( $query )
{
   return $query->whereCompanyId( 1 );
}
```