Add Option<Extractor> to extractors example

[boustrophedon]

It would be nice to mention the fact that in addition to using the FromRequest::extract method, you can also use the implementation of FromRequest for Option<T: FromRequest> if you have form data that may or may not exist.

You usually don't control the data being submitted, so it's pretty much always better to use Option<web::Form> rather than just having actix return "Content type error" to the user.