Refactor CLI & Configuration

[chulkilee]

• Made internal methods private: e.g. parse_options
• Extracted methods: e.g. setup_binds, close_binder_listeners
• Extracted file: dsl.rb for Puma::DSL (originally in configuration.rb)
• Moved Puma::cli_config from configuration.rb to cli.rb
• Reordered methods for readability
• Improved code readability
  • avoid assignment in if condition
  • avoid name conflicts between local variable and instance method
  • shorter block
  • use each_with_object
  • each over each_with_index when index is not used
  • and so on...

Only the first thing (making methods private) will break other apps using them - but from my understanding those methods shouldn't be called outside of puma, so I just made the change.

I started this work as part of #669 and found it would be better to merge refactoring before adding features. Please take a look and please give some feedback.

[evanphx]

Thanks @chulkilee! Great work!