Add very simple execParserMaybe pure parser function for type "Parser a -> String -> Maybe a".

I like the idea of being able to use my option-parsers in a completely IO free manner, making many other approaches than the canonical example possible. Optparse-Applicative was the only library that allowed me to do this that I found, but the code required took a little bit of time to construct.

Let me know if you're interested in this kind of change, if so I might have some more :)