code: function average (arr, options) {
    var isArray = Array.isArray(arr);
    var key = options.hash.key;
    var sum;
  
    if (!isArray) {
      throw new Error('The helper "average" must be passed an Array.');
    }
    if (key) {
      arr = arr.map(function (item) {
        return item[key];
      });
    }
    sum = arr.reduce(function (prev, current) {
      return prev + current;
    });
    return sum / arr.length;
  }
docs: Average an array of numeric values.
  
  @since v0.0.1
  @param {Array} arr
  @return {Number} Returns the average of all values.
  @example
  var numbers = [1, 2, 3];
  var products = [{rating: 1}, {rating: 2}, {rating: 3}];
  
  {{average ratings}} //=> 2
  {{average products key="rating"}} //=> 2