Add an api for customising how verifying doubles are checked

This is a prerequisite for rspec/rspec-rails#1238
Actually, `reference.source` doesn't make sense.  `reference.target` does.  It's the target of the reference, which is used as the source for verification.

So I think `source` would actually probably be the wrong name even though I suggested it.  Let's stick with `target`.

Sorry for the confusion!