contactFormOnBeforeSend()

This hook would allow [Spam Guard](https://github.com/selvinortiz/spamguard) and other plugins to better integrate with **contactform** by allowing processing of submitted data before the email is sent.

Additionally, a plugin can prevent the email from being sent by returning `false` which will tell **contactform** to not send the email but still continue with a successful flow of execution... same behavior  as the honeypot feature.

I know that from a purist standpoint, `contactFormOnBeforeSend` follows an **event** naming pattern rather than a **hook** but it could be changed to anything that makes more sense I suppose!