flex-wrap: wrap-reverse support

Summary:
I couldn't resist to do this 😄 #394

This adds ```flex-wrap: wrap-reverse```

I think we hit a edge case here:
https://stackoverflow.com/questions/33891709/when-flexbox-items-wrap-in-column-mode-container-does-not-grow-its-width

as is differs here from chrome, but I think that yoga is here more correct.

So I haven't added this test yet as this would fail against chrome, as chrome outputs a width of 30 for root, whereas yoga gets a width of 60 here, which I think is correct. Strangely the output of ```flex-wrap:wrap``` is in jsfiddle also only with a (visual) width of 30 on chrome, while the tests gets generated with 60.

```html
<div id="wrap_reverse_column" style="height: 100px; flex-wrap: wrap-reverse">
  <div style="height: 30px; width: 30px;"></div>
  <div style="height: 30px; width: 30px;"></div>
  <div style="height: 30px; width: 30px;"></div>
  <div style="height: 30px; width: 30px;"></div>
</div>
```

Looking forward what you think here emilsjolander
Closes https://github.com/facebook/yoga/pull/398

Reviewed By: astreet

Differential Revision: D4564711

Pulled By: emilsjolander

fbshipit-source-id: 33dc055abd8444b2aa7796ef90bd7ec99e961bb8

gentest/gentest.js

@@ -336,6 +336,7 @@ function overflowValue(e, value) {
 function wrapValue(e, value) {
   switch (value) {
     case 'wrap': return e.YGWrapWrap;
+    case 'wrap-reverse': return e.YGWrapWrapReverse;
     case 'nowrap': return e.YGWrapNoWrap;
   }
 }