Bundler doesn't have a "--with" option

[Vanuan]

As far as I understood, currently bundler do not have opt-in flag, i.e. if I want to include only production group, I'll have to use bundle install --without = all the groups except production. I just want a production group! Why would I bother to list all the groups I don't need? It's a very simple change to implement:

1. If a user specified --with [with_groups]
2. Install gems listed in (default_group + with_groups)

I think, currently bundle install --without follow the following algorithm:

1. If a user specified --without [without_groups]
2. Install gems listed in (default_group + all_groups - without_groups)

I think I could implement this myself

[indirect]

Please see the explanation in #1771.