Feature Request: Configure for Heroku

Since production variables are no longer ignored, there is duplication of effort between `secretive` and `heroku config` when running on Heroku. It seems like poor practice to have both running concurrently.

Perhaps there should be a `Secretive.configure_for_heroku` or `Secretive.ignore_group(:production)`  that will prevent production variables from being set twice on Heroku.
