add .last() to all Arrays

Arrays.prototype.last.js

Array.prototype.last = function(n) {
  if (n === undefined) {
    return this[this.length-1];
  }
  var answers = [];
    for(var i = this.length-1; i>this.length-n-1;i-=1){
      answers.unshift(this[i]);
    }
  return answers;
}