Only prompt for action when running interactively

Hey @pcreux,

This small change to the script only prompts the user for an action _(ie. "keep" or "delete")_ if the script is run interactively.

This is useful when running it as part of a shell script or a cron job, for instance, in which case it will now only print out the orphaned branches, without - indefinitely - waiting for the user to provide a reply.

Thanks, @pvdb