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/fixtures/YGAlignSelfTest.html

@@ -13,3 +13,11 @@
 <div id="align_self_flex_end_override_flex_start" style="width:100px; height: 100px; align-items: flex-start;">
   <div style="height: 10px; width: 10px; align-self: flex-end;"></div>
 </div>
+
+
+<div id="align_self_baseline" style="width: 100px; height: 100px; flex-direction:row;">
+  <div style="width: 50px; height: 50px;align-self: baseline;"></div>
+  <div style="width: 50px; height: 20px;align-self: baseline;">
+    <div style="width: 50px; height: 10px;"></div>
+  </div>
+</div>