Baseline support

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

yoga/YGEnums.h

@@ -104,13 +104,14 @@ typedef enum YGExperimentalFeature {
   YGExperimentalFeatureWebFlexBasis,
 } YGExperimentalFeature;
 
-#define YGAlignCount 5
+#define YGAlignCount 6
 typedef enum YGAlign {
   YGAlignAuto,
   YGAlignFlexStart,
   YGAlignCenter,
   YGAlignFlexEnd,
   YGAlignStretch,
+  YGAlignBaseline,
 } YGAlign;
 
 #define YGUnitCount 3