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/StyleSheet/StyleSheet.js

@@ -13,6 +13,7 @@
 
 var PixelRatio = require('PixelRatio');
 var ReactNativePropRegistry = require('ReactNativePropRegistry');
+var ReactNativeStyleAttributes = require('ReactNativeStyleAttributes');
 var StyleSheetValidation = require('StyleSheetValidation');
 
 var flatten = require('flattenStyle');
@@ -162,6 +163,34 @@ module.exports = {
    */
   flatten,
 
+  /**
+   * WARNING: EXPERIMENTAL. Breaking changes will probably happen a lot and will
+   * not be reliably announced. The whole thing might be deleted, who knows? Use
+   * at your own risk.
+   *
+   * Sets a function to use to pre-process a style property value. This is used
+   * internally to process color and transform values. You should not use this
+   * unless you really know what you are doing and have exhausted other options.
+   */
+  setStyleAttributePreprocessor(property: string, process: (nextProp: mixed) => mixed) {
+    let value;
+
+    if (typeof ReactNativeStyleAttributes[property] === 'string') {
+      value = {};
+    } else if (typeof ReactNativeStyleAttributes[property] === 'object') {
+      value = ReactNativeStyleAttributes[property];
+    } else {
+      console.error(`${property} is not a valid style attribute`);
+      return;
+    }
+
+    if (__DEV__ && typeof value.process === 'function') {
+      console.warn(`Overwriting ${property} style attribute preprocessor`);
+    }
+
+    ReactNativeStyleAttributes[property] = { ...value, process };
+  },
+
   /**
    * Creates a StyleSheet style reference from the given object.
    */
@@ -172,5 +201,5 @@ module.exports = {
       result[key] = ReactNativePropRegistry.register(obj[key]);
     }
     return result;
-  }
+  },
 };