Add onHighlight prop

[chrisforrette]

Hey there. This and the rest of ink has been really rad, thanks for all your hard work!

It would be pretty nice (and helpful for a current project) to have the ability to pass an onHighlight prop (or something like that) that would be invoked when an item is highlighted, e.g. when the up or down arrows are pressed (but not when enter is pressed), and I'd be happy to get a PR together for it if it's something you're open to. Let me know what you think, thanks!

[SimenB]

I'd also like to have this. I wanna print how many items there are left to scroll down to (using limit)

[zimme]

I would suggest this being called onChange.

[zimme]

There's a PR for this here #16 and as I was out voted on the name, it's called onHighlight =)

[vadimdemedes]

Fixed via #16, will keep this issue open until new release is out.