Ability to specify plugins via CLI flags

[Samuel Giddins]

Is your feature request related to a problem? Please describe.

I am unable to specify a plugin when launching puma via the CLI, without a config file

Describe the solution you'd like

puma --plugin $plugin_name

Describe alternatives you've considered

Dynamically writing new config files

Additional context

N/a

[MSP-Greg]

Seems reasonable...

[Nate Berkopec]

I feel like a good way to get your feature request in is to list an alternative you've considered that makes the maintainer go: "ick!"

I'm also in favor, marked as contrib wanted because I feel like the scope here is fairly limited.

[Akshay Birajdar]

I took a shot at this in #3262.