exclude mf.is_resolvable from karma reject capture

so it's deny(soft) can pass through. It provides a mechanism for disabling
rejections.
Except for not rejecting itself - wouldn't it be better to make this configurable in karma.ini e.g.

`````
excludes=access, helo.checks, data.header, spamassassin, mail_from.is_resolvable, clamd
`````

That way anyone with custom plugins can add their own exclusions without hacking the plugin.