Feature/custom classes

First-off, thanks for react-date-picker! I've created this pull request to add the ability to specify custom styling classes per day. My use case is the need to reflect availability information. Being able to block/disable dates that are fully booked for example.

The minor changes made to onSelect and onNav allow me to reason about when I need to fetch availability data and for which periode.

To keep this PR clean I figured I'd only include the changes made to src.

Kind regards, Elger