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
