List available subsystems

[lholmquist]

Sort of related to #58, but this command would list the available subsystems to choose from.

When i was creating a Node core PR, i got a validation error because i used a subsystem that wasn't defined, then i had to go into the code of this repo to find the available ones.

Having the ability to list those values from the core-validate-commit tool would have been nice.

I not sure how the command might look though. maybe something like

core-validate-commit --list subsystem or something

I'm more than happy to work on getting a PR ready for discussion

[richardlau]

I'm more than happy to work on getting a PR ready for discussion

Go for it 😁.