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/entry-common.js

@@ -107,6 +107,9 @@ module.exports = function (bind, lib) {
 
                 case constants.UNIT_PERCENT:
                     return `${this.value}%`;
+                
+                case constants.UNIT_AUTO:
+                    return `auto`;
 
                 default: {
                     return `${this.value}?`;