ADD: group undo/redo support; "Group" button to demo file

[futurist]

There's some need for grouped undo/redo actions, like serial user input, a module that will generate several redo, and then want to undo for the whole module action, etc.

Please see updated demo files, click on Group button, trigger some actions, toggle it, add some more actions, undo...

BTW, is there a need for .editorconfig file? I found it's convinient to maintain same format, and I really spend some time to keep same format as your lib, but still there's some spaces removed by emacs.