Callback for finished migration

[crizzis]

Currently, we can use the callback parameter to Set.up(...) to perform cleanup (e.g. close DB connection).

This does not, however, cover the case when the migration has been performed using the cmdline (if such an option exists, it is undocumented).

It would be nice to be able to specify a 'callback' JS file for when the migration is finished (probably also for when the migration is about to start). Perhaps using the EventEmitter API?