Summary:
Added baseline support (see #132)
You have the ability for a custom baseline function (```float(*YGBaselineFunc)(YGNodeRef node);```) to return whatever baseline you want.
Closes https://github.com/facebook/yoga/pull/317
Reviewed By: splhack
Differential Revision: D4385061
Pulled By: emilsjolander
fbshipit-source-id: cb8a59a09237c840fa3e21753ab68239997dab0c
c169a98be6