Alpha/Reverse Quantile

The functions is `quantile.js` are generic enough that they could support different numerical sorts (reverse) and alpha lists.

```js
const alphaSort = items => items.sort();
const alphaAverage = (a, b) => a + b;
const alpha = quantile(alphaSort, alphaAverage);
```

```js
const reverseSort = items => items.sort((a, b) => b - a)); // reversed!
const reverse = quantile(reverseSort, average);
```