Adds simple rate limiting to the popular Contact Form 7 plugin for WordPress.

• The rate limiting algorithm is very simplistic: it checks to see if the IP address has made 5 or more requests in the past 5 minutes. If it has, the current request is blocked
• No settings page: the 5 minutes and the 5 requests are both hard-coded
• No IPv6 support: an attacker has a bare minimum of 2⁶⁴ source addresses at their disposal, probably more. This plugin should track subnets not addresses when dealing with IPv6
• The generic "spam" message is shown to users instead of a more specific "slow down" message