[DatePicker] Add possibility to customize particular days

[izziaraffaele]

A callback just like shouldDisableDate() to render each day of the calendar would solve the issue and make the DatePicker much more flexible.

This callback ( ex getDayElement() ) must always return a DayButton instance and it's set internally by default. If the callback is passed by props, then the internal callback will be overwritten so that developer can return a customized instance of DayButton from the parent component.

This can also replace completely shouldDisableDate() callback as the developer can set the disable state directly on the DayButton component.

var myDayGetter = function(day){
  // do something with day
  return <DayButton disabled date={day} style={...} />
}

<DatePicker getDayElement={myDayGetter}/>

I see that you're redesigning the component here #3739 so if you like the idea we could integrate it with that...

[mbrookes]

@izziaraffaele #3739 is fixing layout, not trying to fix every DatePicker bug and add every feature requested. 😄

[oliviertassinari]

Closing for #4787