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

gentest/gentest.js

@@ -377,6 +377,7 @@ function alignValue(e, value) {
     case 'stretch': return e.YGAlignStretch;
     case 'flex-start': return e.YGAlignFlexStart;
     case 'flex-end': return e.YGAlignFlexEnd;
+    case 'baseline': return e.YGAlignBaseline;
   }
 }