Custom env and revision

Heroku's staging has production env.
And I want to use this in such staging env.
see https://devcenter.heroku.com/articles/multiple-environments

How to use:
```
    # Enable rack-dev-mark
    config.rack_dev_mark.enable = !Rails.env.production? || ENV['RACK_DEV_MARK']
    # Customize themes if you want to do so
    config.rack_dev_mark.theme = [:title, Rack::DevMark::Theme::GithubForkRibbon.new(position: 'left-bottom', color: 'black', env: ENV['RACK_DEV_MARK'])]
```