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/app/src/chrome/ShareSheetExportUrl.tsx

@@ -16,7 +16,7 @@ import {
   setExportURL,
 } from '../reducers/application';
 import {Logger} from '../fb-interfaces/Logger';
-import {Idler} from '../utils/Idler';
+import {IdlerImpl} from '../utils/Idler';
 import {
   shareFlipperData,
   DataExportResult,
@@ -95,7 +95,7 @@ export default class ShareSheetExportUrl extends Component<Props, State> {
     return this.context.store;
   }
 
-  idler = new Idler();
+  idler = new IdlerImpl();
 
   dispatchAndUpdateToolBarStatus(msg: string) {
     this.store.dispatch(