Making gkx overwrittable

Summary:
**Context:**

We currently have feature flags within our code base that can only be end-to-end tested within our internal environment.

This PR enable us to overwrite feature flag outside our internal environment
Closes #1673

Reviewed By: flarnie

Differential Revision: D7138095

Pulled By: flarnie

fbshipit-source-id: 1b9954cd5c2a16d7c9e4ab4cdb0efceb6e67517a

Author: mitermayer

src/stubs/gkx.js

@@ -14,5 +14,8 @@
 'use strict';
 
 module.exports = function(name: string) {
+  if (typeof window !== 'undefined' && window.__DRAFT_GKX) {
+    return window.__DRAFT_GKX[name];
+  }
   return false;
 };