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

yoga/YGEnums.h

@@ -116,11 +116,12 @@ typedef YG_ENUM_BEGIN(YGPrintOptions) {
   YGPrintOptionsChildren = 4,
 } YG_ENUM_END(YGPrintOptions);
 
-#define YGUnitCount 3
+#define YGUnitCount 4
 typedef YG_ENUM_BEGIN(YGUnit) {
   YGUnitUndefined,
   YGUnitPixel,
   YGUnitPercent,
+  YGUnitAuto,
 } YG_ENUM_END(YGUnit);
 
 #define YGWrapCount 2