prop / func to return what index of the slider you are at?

## I'm looking for robust way of determining whether the slider is at the very-beginning or very-end of the gallery.

I'm currently using the `afterChange` prop, but that returns the index of the particular image it ended at and not where it is on the 'dots'.

I need this functionality to conditionally render my custom arrows depending on the point of the slider / gallery it is in; I can make a PR if this is a feature that is missing