Introduce global environment configuration like `Rails.env`

Since I want to switch dependent gems by environment variables, not by `bundle install` arguments.

This can be by writing wrapper script, and then `Bundler.require(:default, ENV['foobar'])` in it.
But I think this brings little handiness for other developers, if Ruboty has this feature.

How do you think of it? Does Ruboty have any other shortcuts?