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
1146013e9e