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);
```

The issue is about extending quantile.js to support reverse sorting and custom alpha lists. Start by examining the quantile.js file to understand the current quantile function signature and implementation. Look for where sorting and averaging logic is used. The goal is to modify the function to accept a custom sort function and an averaging function as parameters, as shown in the examples. Test the changes by creating examples like the alpha and reverse quantile cases to ensure they work correctly.

索引モデルが issue の本文から書いたものです。