Add onFocus and onBlur methods

[evoactivity]

Fixes #72

[anday013]

@evoactivity Thanks, I'll merge it soon. btw, does it resolve this issue #71 too?

[evoactivity]

I think so, in conjunction with wrapping the otp input with a pressable I use the onBlur method to focus on the pressable when otp loses focus. I don't think there is anything you could do to fix it in this library itself (I tried hacking on the component myself to look for an alternate solution).