Add field for callback to handle unknown/unmapped options during parse

This is the follow-up from our discussion in https://github.com/jessevdk/go-flags/pull/111
Hope this matches what you had in mind. Let me know if you'd like to see any changes.
When I saw that I wanted to comment, "Don't negate the condition like that", then I saw it was already like that, so the shame is on me :) If you can fix, please do so!