selectedDates change does not clear old selected dates

Hi,

Thanks for the lib!
Updating the array of selected dates sets the new dates as selected, but does not clear the old ones that are no longer present in the array.
That's really not intuitive in a React context, where you'd expect the selected dates to always mirror the prop and to not depend on a previous state.
I can submit a PR if you're ok to change that API.
