Add an IMEvent that's fired after config is read

It's sometimes useful to be able to perform actions immediately after config is read but before the user message is processed. An example use case is constructing dynamic states based on configuration, e.g.

* Sandbox starts.
* Load config.
* Hook for creating dynamic states goes here.
* Message processing continues.

This requires adding a new sub-class of `IMEvent` in `state_machine.js` and then firing an event for it if `setup_config` completes successfully.