Use prev/current decision data when updating

[mattrothenberg]

For context: aholachek/react-flip-toolkit#71

Now that the onUpdate function from react-flip-toolkit accepts previous + current decision data as an argument, consumers can more easily write shouldFlip and shouldInvert functions in their Flipped components.

shouldFlip(index) {
  return (prev, current) => {
    // Before, consumers had to cache "prev"
    // somewhere in this component's state
    return index === prev || index === current;
  };
},