Allow multiple classes for the next/prev buttons

It would be nice to specify multiple classes as string within the initialization, something like
```
$(document).foundation({
  orbit: {
      next_class: 'orbit-next fa fa-arrow-left', // Class name given to the next button
      prev_class: 'orbit-prev'
  }
});
```

You can allow multiple classes definition for the next/prev button doing something like this
```
var next_class = settings.next_class.split(" ").join('.');
            var prev_class = settings.prev_class.split(" ").join('.');

            container.on('click', '.'+next_class, self.next);
            container.on('click', '.'+prev_class, self.prev);
```

within the init function