please add an "Undo button" feature

I think this service needs an undo button.  I have used many of the other web editor services and this is a basic thing on all their sites.   I was surprised there was not one.   I think especially if you are new to the service and trying to build things, the Undo button will help immensely as people try to get up to speed with your program.   Please add this as to your list of priority adds to your service.   Thank you.