Add Undo/Redo

[mnsrv]

Resolves #148

• fromUndoRedo is needed to prevent pushing prev color to undo, after we updated current in undo/redo.
• undos stores current as last item, so it never should be empty.
• ~$prev is needed for storing previous color. I tried to use oldValue from subscribe, but because of debounce, it can be the same as value.~
• ~$prev initialised with 0 color to avoid checking for null. I set correct first value in subscribe, when oldValue is not passed~
• ~on cmd + Y in macOS Safari usually opens history, is it okay that now I prevent to open it?~