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/__tests__/api.node.tsx

@@ -59,6 +59,7 @@ test('Correct top level API exposed', () => {
       "DeviceType",
       "Draft",
       "FlipperLib",
+      "Idler",
       "LogLevel",
       "LogTypes",
       "Logger",