Allow puma config file to be authoritative over threads and workers

[nonrational]

Currently when using the following puma config file, the threads and workers options are ignored because the -w and -t flags appear after the -C flag and will override the workers and threads settings. Ideally puma-dev should only supply the workers and threads flags if they have been explicitly defined via environment variables.

Originally expressed via #256
h/t @hasghari