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.
Sorry that I have to nitpick here, but the condition should be done in one line. It is not that complicated to read and additionally it would be the first multiline condition of the project so @jessevdk must say if it is ok and how the code style should look.