Use plural_form to pluralize fruits/deskitems

Summary:
fruits and desk items are now Plural items, so plural_form needs to be
used to pluralize them. I'm not sure if this is the correct approach (as @john
didn't do this before) but it does make the exercise appear correctly.
Sometimes there are errors because "X" and "Y" aren't plural objects, but that
isn't shown to the user and looks fine.

Test Plan:
Go to
http://sandcastle.khanacademy.org/media/castles/xymostech:fix-linear-equations/exercises/interpreting_linear_equations.html?debug&problem=increase&seed=21
See that it doesn't show "undefined" everywhere

Reviewers: eater, alpert, john

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D2359

utils/word-problems.js

@@ -105,6 +105,11 @@ $.extend(KhanUtil, {
 
         var pluralizeWord = function(word) {
 
+            // Check if this is a new Plural object, and just use that plural
+            if (word && word.plural) {
+                return word.plural(2);
+            }
+
             // noone really needs extra spaces at the edges, do they?
             word = $.trim(word);
 
@@ -175,7 +180,8 @@ $.extend(KhanUtil, {
                 }
 
                 return value + " " + arg1;
-            } else if (typeof value === "string") {
+            } else if (typeof value === "string" || typeof value === "object") {
+                // We need to accept objects here as well for new Plural objects
                 var plural = pluralizeWord(value);
                 if (typeof arg1 === "string" && arguments.length === 3) {
                     plural = arg1;