text: Title: Disable multiple dates/date types in dcc.DatePicker*
  Body: I'll open this here but I am aware that you're using [AirBnB's react-dates](https://github.com/airbnb/react-dates/issues) so I'll start here and then open an issue there if it's not something that can be implemented on the dash side.
  
  We'd like to disable "categories" of days from being selected. Our smallest request would be disabling weekdays or weekends, but generally supplying a list of available dates or unavailable dates to the DatePicker would be :+1: 