Undum option: repeatable actions

A repeatable action is an action which can be called more than once.

But what if a game has no repeatable actions? Every link in the game still has to bear "once" class.
So I've implemented a switch for Undum. Just call:

    system.options.repeatable_links = false

in your `undum.game.init` function and get rid of all "once" classes! It completely turns off repeatable actions.

I don't think the option name is really the best choice but I definitely need this feature.