Add LooseSignatureVerifier

See https://github.com/rspec/rspec-mocks/pull/726
Maybe I'm misunderstanding what you mean by "extra" but `extra_kw_check` is reversed from how I would expect it to behave.  I'd expect `extra_kw_check` to check keyword args when true and not check it when false but it looks like it does the opposite?

(Or maybe the `unless` combined with a compound conditional is confusing me and I'm tired.  Regardless, I'm confused by this at the moment at least).