“Back” button for step-by-step config

[kizu]

If I chose an incorrect option at http://csscomb.ru/config and then wanted to go back, right now there is no way to do it.

Also: going back in history resets the config: it would be handy to store the unfinished state somehow, so if you wouldn't finish the config, then you could come back and continue it from the place you left it.