lib/plugins/stats.js
function sum(arr, prop){
  return arr.reduce(function(sum, obj){
    return sum + obj.stats[prop];
  }, 0);
}
function sum(arr, prop){
  return arr.reduce(function(sum, obj){
    return sum + obj.stats[prop];
  }, 0);
}
Return sum of each `prop` in `arr`.

@param {Array} arr
@param {String} prop
@return {Number}
@api private