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/createTablePlugin.tsx

@@ -24,7 +24,7 @@ import {List, Map as ImmutableMap} from 'immutable';
 import React from 'react';
 import {KeyboardActions} from './MenuBar';
 import {TableBodyRow} from './ui';
-import {Idler} from './utils/Idler';
+import {Idler} from 'flipper-plugin';
 
 type ID = string;