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/Node.hh

@@ -176,6 +176,9 @@ class Node {
 
     Layout getComputedLayout(void) const;
 
+    double getComputedMargin(int edge) const;
+    double getComputedPadding(int edge) const;
+
  private:
 
     YGNodeRef m_node;