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/YGEnums.js

@@ -86,10 +86,11 @@ module.exports = {
   PRINT_OPTIONS_STYLE: 2,
   PRINT_OPTIONS_CHILDREN: 4,
 
-  UNIT_COUNT: 3,
+  UNIT_COUNT: 4,
   UNIT_UNDEFINED: 0,
   UNIT_PIXEL: 1,
   UNIT_PERCENT: 2,
+  UNIT_AUTO: 3,
 
   WRAP_COUNT: 2,
   WRAP_NO_WRAP: 0,