Added option to emphasize some featured dates

[FabianScheidt]

I needed a way to emphasize some days in the calender so I extended the plugin to "feature" some dates using a css class.
If you think this is useful, I'd be happy if you accept this pull request. I wrote unit tests and extended the documentation for this feature.

[renesteeman]

I was just looking for that functionality. It would indeed be a great addition.

[chtheis]

I was looking into user defined classes, too: people book accommodation for an event and in the calendar I want to mark the period the event takes place. But instead of adding more and more classes to the code base I pass the date to the method createItem and inside that method I call an optional callback onCreateItem which returns an array with the classes to set.