Additional Improvements & Options with CodeGen + Dropping Ruby 1.9

Hi. I'm not sure if the Twitch team still maintains this package, but I wanted to suggest (and potentially help) with a couple of changes to the Code Generator plugin.

- There should be an options to generate only Services, or Clients (not both)
- There should be options for Module/class naming
- There should be options for the relative filenames that Twirp expects (from `protoc --ruby_out`)

Let me know if any of this is planned!

Thanks in advance

EDIT: I was also wondering if there are any plans to drop Ruby 1.9.

Start by locating the Code Generator plugin and its handling of protoc --ruby_out output. The issue covers selective Services/Clients generation, module/class naming, relative filenames, and possibly dropping Ruby 1.9; done requires a decided scope and agreed behavior for those options.

Written by the indexing model from the issue text.