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?
momoize not to be changed after initialization.