Value API & Relative Scrolling

[ganderzz]

Add the ability to scroll by a relative amount (ie. if the window is scrolled down by 100px, we should be able to scroll by -50px to end at 50px scrolled)

This is resolved:
#79

---

The way this is handled also gives rise to extensible value handlers. An example with the new relative() function:

<ScrollTo>
   {({ scrollTo, relative }) => (
       <button onClick={() => scrollTo({ y: relative(-50) })}>Click Me</button>
   )}
</ScrollTo>

Looking at the implementation of relative(): https://github.com/ganderzz/react-scroll-to/pull/79/files#diff-cfc450c9626bea8788ec583b684d167b

Users could add their own functions:

export function addTen(input) {
  if (!input || typeof input !== "number") {
    console.warn(
      `Invalid input (${input}) given to relative(). Is this a number?`
    );
  }

   return function(element, isHorizontal) {
    if (!element || !isHorizontal) {
      return input;
    }

     return input + 10;
  };
}

<ScrollTo>
   {({ scrollTo, relative }) => (
       <button onClick={() => scrollTo({ y: addTen(20) })}>Click Me</button>
   )}
</ScrollTo>

^ Will scroll down by 30px