Add an implementation of Animated.subtract

Summary:
Fixes #18451

I've added another example to NativeAnimationsExample, which makes use of `Animated.substract()`, let me know if the example is not desired / doesn't add much value. Below two GIFs of the new method working on iOS and Android:

<img width="320" src="https://user-images.githubusercontent.com/1437605/38154748-165cc5f8-3474-11e8-8b31-504444271896.gif" />
<img width="320" src="https://user-images.githubusercontent.com/1437605/38154749-1679bff0-3474-11e8-80b1-b558d44e0494.gif" />

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facebook/react-native-website#276

[GENERAL] [ENHANCEMENT] [Animated] - Implemented Animated.subtract
Closes facebook/react-native#18630

Differential Revision: D7462867

Pulled By: hramos

fbshipit-source-id: 4cb0b8af08bb0c841e44ea2099889b8c02a22a4a

js/NativeAnimationsExample.js

@@ -429,6 +429,48 @@ exports.examples = [
       );
     },
   },
+  {
+    title: 'Multistage With Subtract',
+    render: function() {
+      return (
+        <Tester type="timing" config={{duration: 1000}}>
+          {anim => (
+            <Animated.View
+              style={[
+                styles.block,
+                {
+                  transform: [
+                    {
+                      translateX: anim.interpolate({
+                        inputRange: [0, 1],
+                        outputRange: [0, 200],
+                      }),
+                    },
+                    {
+                      translateY: anim.interpolate({
+                        inputRange: [0, 0.5, 1],
+                        outputRange: [0, 50, 0],
+                      }),
+                    },
+                  ],
+                  opacity: Animated.subtract(
+                    anim.interpolate({
+                      inputRange: [0, 1],
+                      outputRange: [1, 1],
+                    }),
+                    anim.interpolate({
+                      inputRange: [0, 0.5, 1],
+                      outputRange: [0, 0.5, 0],
+                    }),
+                  ),
+                },
+              ]}
+            />
+          )}
+        </Tester>
+      );
+    },
+  },
   {
     title: 'Scale interpolation with clamping',
     render: function() {