toggle verbosity

https://github.com/rwjblue/ember-module-migrator/blob/8ddb85d6e311d3cbe32e2bc5c70d475864392b3a/bin/ember-module-migrator#L23

On big projects, this list isn't that handy. But rendering it is slower then the whole migration process.

Maybe we should by default emit a file with this information ? might be nice to consult it at a later date if at all.
