index.js
function avg(arr, idx, range) {
  return sum(arr.slice(idx - range, idx)) / range;
}
function avg(arr, idx, range) {
  return sum(arr.slice(idx - range, idx)) / range;
}
function avg ( arr , idx , range ) { return sum ( arr . slice ( idx - range , idx ) ) / range ; }
Create an average for the specified range.

```js
console.log(avg([1, 2, 3, 4, 5, 6, 7, 8, 9], 5, 4));
//=> 3.5
```
@param  {Array} `arr` Array to pull the range from.
@param  {Number} `idx` Index of element being calculated
@param  {Number} `range` Size of range to calculate.
@return {Number} Average of range.
Create an average for the specified range .