Introduce onDestroy hook

Summary: This diff introduces the `onDestroy` hook that can be used by plugins to listen to the event where a plugin is cleaned up (either because it is disabled, or because the client is being cleaned up)

Reviewed By: jknoxville

Differential Revision: D22208121

fbshipit-source-id: 9c4951ae671be611f21da171c548d4054c481166

desktop/flipper-plugin/src/index.ts

@@ -8,3 +8,4 @@
  */
 
 export * from './plugin/Plugin';
+export * from './plugin/SandyPluginDefinition';