added support for optional gem groups

[briri]

Adds support for including optional gem groups: https://bundler.io/guides/groups.html
The default for this new argument is nil.

For example if the following is in the Gemfile:

group :rollbar , optional: true do
  gem 'rollbar'
end

You can add set :bundle_with, %w{ rollbar }.join(' ') to your deploy.rb file(s)

So that bundler runs as bundle install --with rollbar

[capistrano-bot]

Thanks for the PR! This project lacks automated tests, which makes reviewing and approving PRs somewhat difficult. Please make sure that your contribution has not broken backwards compatibility or introduced any risky changes.

Generated by 🚫 Danger

[briri]

Ran tests against this code which resulted in:

00:14 bundler:install
      01 bundle install --path /my/application/shared/bundle --jobs 4 --with aws_s3 --without pgsql development test --deployment --quiet

[manno]

I was able to work around this by using set :bundle_config, { with: 'productionplus' }