Add an optional index argument to _.last()

This makes _.last() behave the same as _.first().  Passing an optional
second argument n will return the last n elements of the array.

test/arrays.js

@@ -26,6 +26,8 @@ $(document).ready(function() {
 
   test("arrays: last", function() {
     equals(_.last([1,2,3]), 3, 'can pull out the last element of an array');
+    equals(_.last([1,2,3], 0).join(', '), "", 'can pass an index to last');
+    equals(_.last([1,2,3], 2).join(', '), '2, 3', 'can pass an index to last');
     var result = (function(){ return _(arguments).last(); })(1, 2, 3, 4);
     equals(result, 4, 'works on an arguments object');
   });

underscore.js

@@ -306,9 +306,10 @@
     return slice.call(array, (index == null) || guard ? 1 : index);
   };
 
-  // Get the last element of an array.
-  _.last = function(array) {
-    return array[array.length - 1];
+  // Get the last element of an array. Passing **n** will return the last N
+  // values in the array.
+  _.last = function(array, n) {
+    return (n != null) ? slice.call(array, array.length - n) : array[array.length - 1];
   };
 
   // Trim out all falsy values from an array.