Summary: This diffs adds the capability to listen to messages in Sandy plugins. Although API wise it looks more like the old `this.subscribe`, semantically it behaves like the `persistedStateReducer`; messages are queued if the plugin is enabled but not active.
Reviewed By: nikoant
Differential Revision: D22282711
fbshipit-source-id: 885faa702fe779ac8d593c1d224b2be13e688d47
