Allow custom platforms for the RN Packager on a per-project basis

Reviewed By: cpojer

Differential Revision: D4255979

fbshipit-source-id: bf900b67ee30e2f994e96c9a6103ed2e53a87f88

local-cli/default.config.js

@@ -34,6 +34,15 @@ var config = {
     return [];
   },
 
+  /**
+   * Specify any additional platforms to be used by the packager.
+   * For example, if you want to add a "custom" platform, and use modules
+   * ending in .custom.js, you would return ['custom'] here.
+   */
+  getPlatforms() {
+    return [];
+  },
+
   /**
    * Returns a regular expression for modules that should be ignored by the
    * packager on a given platform.