Before and after versions of the safemigrate command

[ryanhiebert]

We could add exclusive flags --before-deploy and --after-deploy to provide some additional benefits when we know that we have an automated command that will run after deployment, rather than being limited to only immediately before deployment.

Particularly, it would be optimal if the delay for an after_deploy migration didn't begin until the deployment had completed. This isn't a show-stopping issue for usage, but it would clean up the semantics an important little bit.