Add support for async / custom plugin export

Summary:
Sandy plugins can now set up an `onExport` handler to enable customizing the export format of a plugin: `client.onExport(callback: (idler, onStatusMessage) => Promise<state>)`

Import will be done in next diff

Reviewed By: nikoant

Differential Revision: D26124440

fbshipit-source-id: c787c79d929aa8fb484f15a9340d7c87545793cb

desktop/flipper-plugin/src/index.ts

@@ -69,6 +69,7 @@ export {
   useLogger,
   _LoggerContext,
 } from './utils/Logger';
+export {Idler} from './utils/Idler';
 
 // It's not ideal that this exists in flipper-plugin sources directly,
 // but is the least pain for plugin authors.