Feature wait for background to exit

[schneems]

Either add a flag to background.stop or introduce a new command.

Like background.stop(wait_exit: true) might need to add a timeout as well to ensure no deadlock. This would help for heroku run:inside <dyno> "rake db:migrate" where we know it will exit and want to wait for that. Actually, need to double check that the run inside exits when the high level command exits