Add StyleSheet.setStyleAttributePreprocessor

Summary:
**Motivation**

On Exponent we load fonts dynamically and assign their native names by appending a session id, so that fonts from one Exponent "experience" do not clash with each other. So, before sending the `fontFamily` to native, we want to change it to the Exponent-scoped `fontFamily`.

Example:

```js
// Before rendering your app
StyleSheet.setStyleAttributePreprocessor('fontFamily', _processFontFamily);

function _processFontFamily(name) {
  // Pass system fonts through
  if (!name || Constants.systemFonts.indexOf(name) >= 0) {
    return name;
  }

  if (!Font.isLoaded(name)) {
    if (__DEV__) {
      console.error(`${name} is not a system font and has not been loaded through Exponent.Font.loadAsync. If you intended to use a system font, make sure you typed the name correctly and that it is supported by the current operating system. If this is a custom font, be sure to load it with Exponent.Font.loadAsync`);
    } else {
      return 'system';
    }
  }

  return `ExponentFont-
Closes https://github.com/facebook/react-native/pull/11138

Differential Revision: D4245518

Pulled By: mkonicek

fbshipit-source-id: bd2452b1129d6675aa7b88e41351f8bb61fa20a3

Libraries/Renderer/src/renderers/native/__tests__/ReactNativeAttributePayload-test.js

@@ -0,0 +1,22 @@
+jest.disableAutomock();
+
+const ReactNativeAttributePayload = require('ReactNativeAttributePayload');
+const ReactNativeStyleAttributes = require('ReactNativeStyleAttributes');
+const StyleSheet = require('StyleSheet');
+
+describe('ReactNativeAttributePayload', () => {
+
+  describe('create', () => {
+    it('works with custom style processors', () => {
+      StyleSheet.setStyleAttributePreprocessor('fontFamily', (nextValue) => 'Wingdings');
+
+      const updatePayload = ReactNativeAttributePayload.create(
+        {style: {fontFamily: 'Comic Sans'}},
+        {style: ReactNativeStyleAttributes},
+      );
+
+      expect(updatePayload.fontFamily).toEqual('Wingdings');
+    });
+  });
+
+});