Accessing old value in update event

[xeruf]

I would like to check on update whether the value increased or decreased. There does not seem a way to do this without tracking state separately, as this.get() already returns the new value.

Furthermore, it would be good to mention in https://refreshless.com/nouislider/events-callbacks/#section-binding that this won't be set if you use a lambda:

        slider.noUiSlider.on('update', (values, handle) => {
          console.log(this)
        })

While this works:

        slider.noUiSlider.on('update', function(values, handle) {
          console.log(this)
        })