Enable specific days

[landsman]

This is follow-up to already existing: #1584

When you have turn on property showOutsideDays there is no easy way how to achieve this design from outside I think.

Hard way would be to generate "occupied days" for specified month with some "drop off" before and after, which is kinda overkill.
So I can imagine some kind of property to switch this logic from "disabled" to "allowed".

What do you think?

[gpbl]

@landsman Would you replicate your problem in https://codesandbox.io/s/react-daypicker-vl4ry? Thanks!

[landsman]

I tried and found bug in my code, sorry for that!