Feature request: Form Submission Throttling

[timbertens]

Fighting spam is always a pain and you have already a set of options to do so. We also try to reduce the impact for 'normal' website visitor as much as possible, so Recaptcha is very effective, but our last resort mainly. What we found is that Submission Throttling (together with honeypot) is pretty effective to avoid continuous form submissions from a bot, I believe it would be a good addition to your plugin