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

javascript/sources/YGEnums.js

@@ -76,12 +76,13 @@ module.exports = {
   EXPERIMENTAL_FEATURE_ROUNDING: 0,
   EXPERIMENTAL_FEATURE_WEB_FLEX_BASIS: 1,
 
-  ALIGN_COUNT: 5,
+  ALIGN_COUNT: 6,
   ALIGN_AUTO: 0,
   ALIGN_FLEX_START: 1,
   ALIGN_CENTER: 2,
   ALIGN_FLEX_END: 3,
   ALIGN_STRETCH: 4,
+  ALIGN_BASELINE: 5,
 
   UNIT_COUNT: 3,
   UNIT_UNDEFINED: 0,