Add the ability to configure a custom Taggables model class

[eggnaube]

This change allows the use of an own taggables class. Thus, for example, events can be easily registered on the taggables.

[freekmurze]

Thanks!

[gregpriday]

I can submit a separate PR for this, but shouldn't we ensure some sort of backward compat with old config files to prevent issues like this #472?

Something like this:

public function getPivotModelClassName(): string
{
    return config('tags.taggable.class_name', Illuminate\Database\Eloquent\Relations\MorphPivot::class);
}