Adds airbrake_stages and airbrake_role configurations.

In an environment I'm working in we have multiple app servers in our environment and use dev and staging stages before production. We really only want Airbrake to run notifications in production and only on one server (not for every app server) per deploy. I imagine other people would benefit from these capabilities.

I believe the way I have written these changes basic usage of Airbrake Capistrano v3 integration will operate exactly as it does now. I have attempted to merely add further ability to limit when and how much Airbrake will be applied :)

I'll happily contribute further by updating the relevant documentation if my pull request is accepted.