expose userConfig for useImperativeHandle methods

[Zoltan Peto]

It would be really nice if we could configure this (current behaviour is too fast/too snappy I think):

const userConfig = {
  damping: 15,
  stiffness: 120,
  mass: 0.5,
  overshootClamping: false,
  restDisplacementThreshold: 0.001,
  restSpeedThreshold: 0.001,
};