Feature: support Undo and redo in the demo

[cmcaine]

Users of WYSIWYG editors expect undo and redo buttons and keyboard shortcuts to exist and work. The demo doesn't include those, so it's not clear if those are easy features to support when embedding email-builder-js.

[ibnsultan]

you may take a look at this and see if it works per your needs https://github.com/ibnsultan/emailBuilder.js