[Feature Request] able to add custom transform to helper element

[DanSnow]

Description

Thanks for the awesome package. We are using this package in our application and we want to add a rotate effect when dragging. We use helperClass to add a custom class for this. But when I started to move my cursor, the effect is immediately gone. I found that it's because here use translate to move the ghost element and override our transform effect.
https://github.com/Jexordexan/vue-slicksort/blob/main/src/ContainerMixin.js#L564-L566

Is it possible to add a prop to let us append some custom styles?

Expected result

can add custom transform

Actually result

transform overrides by the package

Proposal solution

1. add a prop like set-helper-style and accept a function to allow overriding the part of updating styles
2. add a prop like helper-custom-transform and append to the transform style