beforeShowDay callback within options object

This callback is for the purpose of enabling, disabling, or applying CSS classes to specific dates in the calendar. During the while loop that configures each day, it passes the iterated date to the callback to be checked.

The signature of the callback is as follows:
```javascript
function(date:Date):Array
```

```javascript
// This will disable the date being checked
[false]

// This will disable the date being checked and apply the 'special-date' css class to the date
[false, 'special-date']

// This will enable the date and apply the 'special-date' css class to the date
[true, 'special-date']
```