New command `revoke-userid`

There's a certify-userid subcommand but it seems there's no way to revoke that certification in sop.

I propose to add such a command which would vastly simplify our existing code using GnuPG.

If this sounds like a good idea I'm happy to write a draft mimicking certify-userid (but if it's not a good idea I'd rather not do that now 😅 )