Add Copy Constructor for C# YogaNode

Summary:
Add Copy Constructor for convenience.

    YogaNode defaultStyle = new YogaNode
    {
      Width = 100,
      Height = 100,
    };

    YogaNode node0 = new YogaNode(defaultStyle)
    {
        FlexDirection = YogaFlexDirection.Row,
        JustifyContent = YogaJustify.FlexEnd,
    };

    YogaNode node1 = new YogaNode(defaultStyle)
    {
        AlignItems = YogaAlign.Center,
    };

Reviewed By: emilsjolander

Differential Revision: D4393234

fbshipit-source-id: b4b73071aab4dc1b3ae422969de659380bd0e3ad

csharp/Facebook.Yoga/YogaNode.cs

@@ -36,6 +36,12 @@ namespace Facebook.Yoga
             }
         }
 
+        public YogaNode(YogaNode srcNode)
+            : this()
+        {
+            CopyStyle(srcNode);
+        }
+
         public void Reset()
         {
             _measureFunction = null;