Add some ability to manually bypass checking some messages

Gmail is the largest free mail provider, and as such, their IPs tend to be marked for sending a lot of spam. Usually this is no problem, but this can cause issues with legitimate mail being blocked, for example, sometimes when Gmail attempts to forward me something as part of a forwarding rule I've set up. Therefore it would be useful to be able to configure custom whitelisting, to allow the check script to pass (DUNNO) messages that match, preventing them from being blocked.

I could see this as three configuration values:

• Sender whitelist (emails from this address are not checked)
• Recipient whitelist (emails to this address are not checked)
• IP block whitelist (any client address matching an IP or IP block is not checked)