Adds onDayPickerShow to complement onDayPickerHide

[kylekthompson]

👋 Hello.

While working with react-day-picker, I ran into a situation where I wanted to know whether the day picker was open. To help with figuring that out, this PR exposes an onDayPickerShow callback to complement the onDayPickerHide callback that already exists.

Typically I'd instead opt for keeping the showOverlay state local to my own component and use this as a controlled component, but it doesn't look like that is possible (if we'd rather have the ability to do that, I'm willing to open a PR or modify this one to support that instead, but I think we'd still need this callback to know when to set that state to true)

[codecov]

Codecov Report

Merging #831 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #831   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files          15     15           
  Lines         645    647    +2     
  Branches      141    142    +1     
=====================================
+ Hits          645    647    +2

Impacted Files	Coverage Δ
src/DayPickerInput.js	100% <100%> (ø)	⬆️

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[gpbl]

Good idea, thanks!