Undo/redo functionality - what would be the best way to implement it on top of pure-store?

Hey @gunn, 

pure-store looks quite good for a small wrapper on top of immer. 
Did you ever think about having undo/redo baked in into it? 

I have implemented a small class, that is basically a copy of `PureStore` with undo/redo included into it, would you be interested in a contribution? 

Cheers
Roman