Add useful undo/redo parameters to go N back/forward or go to N step.

[wbern]

To save some performance, I wish I could do that.

[caugner]

@wbern Due to a limitation of Vue/Vuex, no performance would actually be saved: https://github.com/vuejs/vuex/issues/651

Vuex-Undo-Redo still has to dispatch each mutation, and each mutation triggers the reactivity of each affected component. This makes "undo" very slow for higher number of steps.