request: Add a --skip subcommand

[kleimkuhler]

Motivation

--feature-powerset was added recently and it has been really helpful.
Unfortunately, if there is a feature that doesn't need to be tested in the
combinations it's not possible to skip. Each additional feature that is tested
in the powerset doubles the run-time so it can be costly as the number of
features grows.

Solution

Add a --skip subcommand that can be paired with --each-feature or
--feature-powerset. The latter subcommands would filter out any features
passed in --skip before running their own logic.

I wanted to open the issue before sending in a PR incase @taiki-e had any
thoughts? If this is something you are okay with adding, I can send in a quick
change to add it. It should be fairly straightforward.

[taiki-e]

Seems reasonable to me!