I think we should accept environment variables for these values too when not in a darwin machine.

See:

https://github.com/boxen/boxen/blob/master/lib/boxen/config.rb#L260
https://github.com/boxen/boxen/blob/master/lib/boxen/config.rb#L316
https://github.com/boxen/boxen/blob/master/lib/boxen/config.rb#L325