Add YGLayoutGetMargin

Summary:
Fix #326. I'll open another PR once this one gets accepted to add support for `YGLayoutGetBorder` 👌
Closes https://github.com/facebook/yoga/pull/335

Reviewed By: gkassabli

Differential Revision: D4409399

Pulled By: emilsjolander

fbshipit-source-id: 8153f6701cab60b55a485f6d2e0b9f7767481090

javascript/sources/nbind.cc

@@ -153,4 +153,7 @@ NBIND_CLASS(Node)
     method(getComputedHeight);
 
     method(getComputedLayout);
+
+    method(getComputedMargin);
+    method(getComputedPadding);
 }