KeyBindingUtil: add usesMacOSHeuristics method (#869)

Summary:
**Summary**

Add a public method, `usesMacOSHeuristics`, to the `KeyBindingUtil` module. This makes it possible to determine how draft-js will interpret certain keyboard events and thus alter the UI accordingly, e.g.;

<img width="499" alt="screenshot 2016-12-13 at 17 58 33" src="https://cloud.githubusercontent.com/assets/6705160/21150013/dee64566-c15d-11e6-9639-75a39d21d079.png">

I currently achieve this by doing (🙈);

```js
const COMMAND_MODIFIER_CHAR = (navigator && navigator.userAgent.indexOf('Mac') >= 0)
    ? '\u2318' // Mac command key character
    : 'Ctrl'
```
Pull Request resolved: facebookarchive/draft-js#869

Differential Revision: D9734962

fbshipit-source-id: ed3f3d0a87df51c708fd666fe85bc6acba1ecfee

docs/APIReference-KeyBindingUtil.md

@@ -28,6 +28,15 @@ isOptionKeyCommand: function(
 ): boolean
 ```
 
+### usesMacOSHeuristics
+
+```
+usesMacOSHeuristics: function(): boolean
+```
+
+Check whether heuristics that only apply to macOS are used internally, for
+example when determining the key combination used as command modifier.
+
 ### hasCommandModifier
 
 ```

src/component/utils/KeyBindingUtil.js

@@ -31,6 +31,10 @@ const KeyBindingUtil = {
     return isOSX && e.altKey;
   },
 
+  usesMacOSHeuristics: function(): boolean {
+    return isOSX;
+  },
+
   hasCommandModifier: function(e: SyntheticKeyboardEvent<>): boolean {
     return isOSX
       ? !!e.metaKey && !e.altKey