Add compatibility with sopel-rep's karma commands

[dgw]

This is a small step beyond just validating nicks against the character set allowed. sopel-rep's karma commands use ++ and -- appended to a nickname. + is not valid in nicks. The issue is deciding whether to bomb nick-- or nick, probably by checking if nick-- is present and falling back to nick if not. (Which means .bomb nick--- would also work to downrep and bomb nick- at once.)

I've proved the idea of simply importing the sopel-rep module's verification function in tests using sopel-duel, so that's probably the way to go. (I might want to rewrite the rep module using proper encapsulation before doing this, though.)