Summary:
This adds some improvements to the new ```YGConfig```, it tackles #452 and moves the scalefactor into the config.
Closes https://github.com/facebook/yoga/pull/457
Differential Revision: D4675088
Pulled By: emilsjolander
fbshipit-source-id: 99b2c734d6c5139fe1dc8bdeb014bb038f0e337d