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.
This bugs me for some time now. I do not know how @jessevdk feels about it but this is the only enclosing negation in the project, please remove it.