Adds afterTagSave callback to set tag classes properly.

[steventebrinke]

Changing the colour of tags by setting a class did not work when tags were unchanged after editing ended with the separator, because in that case the DOM element was recreated, but no change event was fired since the tags were unchanged. IMO, not firing a change event when tags are unchanged is correct, so onChange is not suitable for adding classes to tags. Therefore, I added an afterTagSave callback that receives the DOM element of the saved tag, which makes it easier to add custom CSS classes. The demo has been updated accordingly.