Add Linear easing to AnimationUtils

Summary:
Simple linear easing also very useful.
Closes https://github.com/facebook/react-native/pull/794
Github Author: Ivan Starkov <istarkov@gmail.com>

Test Plan: Imported from GitHub, without a `Test Plan:` line.

Libraries/Animation/AnimationUtils.js

@@ -20,6 +20,9 @@
 type EasingFunction = (t: number) => number;
 
 var defaults = {
+  linear: function(t: number): number {
+    return t;
+  },
   easeInQuad: function(t: number): number {
     return t * t;
   },