DayPickerInput's OverlayComponent should accept ref

[gzzo]

To create a responsive day picker, we need to have a ref to the OverlayComponent so we can set it's left or right attribute such that it doesn't open off screen.

I see two approaches:

1. Explicit overlayRef prop.
2. Add a overlayProps prop, and pass {...overlayProps}.

Either of these would be implemented here. I'm happy to open a PR to implement this, just wanted to get some thoughts on the preferred approach here.