feat(coverflow) : allow function modifier

[Grafikart]

Why

For a project I need to have a coverflow effect that only zoom on the current Item. The coverflow effect is nice but it's not possible to lessen the scaling effect. With this proposal a custom function can be used as a modifier to handle more complex scaling for the coverflow effect.

Use case

For instance :

modifier: (v) => between(1, -1, Math.round(v))

Would create a scaling that won't affect the elements further they are from the center.