Feature auto margin

Summary:
Even so I know there are some opinions against ```margin: 0 auto``` it's still part of the spec: https://www.w3.org/TR/css-flexbox-1/#auto-margins and pretty usefull if you have to position via ```justify-content```.

This PR adds an implementation for that.

It adds an additonal ```YGUnitAuto``` and margins got ```YGNodeStyleSetMarginAuto``` functions as well.
Closes https://github.com/facebook/yoga/pull/357

Reviewed By: astreet

Differential Revision: D4501142

Pulled By: emilsjolander

fbshipit-source-id: 86519f8632496f46e78a7c9dbc5b21e212e3e0c7

javascript/sources/Node.cc

@@ -119,6 +119,11 @@ void Node::setMarginPercent(int edge, double margin)
     YGNodeStyleSetMarginPercent(m_node, static_cast<YGEdge>(edge), margin);
 }
 
+void Node::setMarginAuto(int edge)
+{
+    YGNodeStyleSetMarginAuto(m_node, static_cast<YGEdge>(edge));
+}
+
 void Node::setOverflow(int overflow)
 {
     YGNodeStyleSetOverflow(m_node, static_cast<YGOverflow>(overflow));
@@ -164,6 +169,11 @@ void Node::setWidthPercent(double width)
     YGNodeStyleSetWidthPercent(m_node, width);
 }
 
+void Node::setWidthAuto()
+{
+    YGNodeStyleSetWidthAuto(m_node);
+}
+
 void Node::setHeight(double height)
 {
     YGNodeStyleSetHeight(m_node, height);
@@ -174,6 +184,11 @@ void Node::setHeightPercent(double height)
     YGNodeStyleSetHeightPercent(m_node, height);
 }
 
+void Node::setHeightAuto()
+{
+    YGNodeStyleSetHeightAuto(m_node);
+}
+
 void Node::setMinWidth(double minWidth)
 {
     YGNodeStyleSetMinWidth(m_node, minWidth);