Option to remove duplicates #25

Currently the classes are simply prepended to the class attribute with a preg_replace.
I've created a method in my order (similar) plugin that can handle duplicates and both qouble and single quotes.
See: https://github.com/JoryHogeveen/genesis-widget-column-classes/blob/1.2.2/genesis-widget-column-classes.php#L389-L458

Lets patch it over along with it's unit tests!