Adds support for using puma config file in capistrano deploys.

* Unless `puma_config_file` given `./config/puma/<env>.rb` is used.
* Fallback to using `puma_*` variables if config file cannot be found.
* If path to state file is not given then then `fetch(:puma_state)` is used.

Seems to work well in both modes. Any suggestions are more than welcome!