Support for using RecaptchaVerificationHelper from ActionFilter

[F0b0s]

Hello, thanks for great lib!
I would like to use it in my project, but suddenly found that RecaptchaVerificationHelper is available only in controller.
It seems to me captcha verification is cross-cutting concern and can be implemented as ActionFilter, and it can improve your library, can you add support for this funcrionality? Or i can help you and create a pull request.