code: function avg(arr, idx, range) {
    return sum(arr.slice(idx - range, idx)) / range;
  }
docs: 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.