Do not apply sluggable when slug attribute is set explicitly

Maybe it should be like an option in configuration, but I think tweaking the `needsSlugging` method is reasonable for disable auto-slugging when an attribute is set explicitly. For example, if I set `slug` in my form for Post, I do not want to `title` be slugged by the package. On the other hand, if I didn't set `slug`, I want to my `title` be slugged.