Add a way to run _only_ experimental checks

[autarch]

Description

I'd like to be able run just the notowned check. I tried to do that this way:

CHECKS="" EXPERIMENTAL_CHECKS="notowned" codeowners-validator

However, it seems like if CHECKS is an empty string, then all the checks are run. This was a bit surprising.

There doesn't seem to be a way to disable all of the default checks.

Reasons

This is related to #225. I want to run the notowners check in CI, but not in a pre-commit hook, so I've set things up to run everything besides notowners as a pre-commit hook.