storing information about failed plugins

Summary:
If a plugin was not loaded we used to ignore it. This diff still ignores the plugins that weren't loaded, but adds some information about them to the redux store. This information is used in the PluginDebugger.

These are the three reasons, why a plugin might not be loaded.

* `gatekeepedPlugins`: Plugin was ignored because of a GK
* `failedPlugins`: Plugin could not be requried/failed to parse
* `disabledPlugins`: Plugin disabled in `~/.flipper/config.json`

Information for each them is added to the redux store.

Reviewed By: passy

Differential Revision: D13516986

fbshipit-source-id: b7a55a159cb586d1a88fbb976248131c52a909c5

src/plugin.js

@@ -52,6 +52,8 @@ export class FlipperBasePlugin<
   static title: ?string = null;
   static id: string = '';
   static icon: ?string = null;
+  static gatekeeper: ?string = null;
+  static entry: ?string = null;
   static bugs: ?{
     email?: string,
     url?: string,