Allow specifying rules to keep unmanaged

[timbuchwaldt]

When using iptables_raw with a system that injects it's own firewall rules like Docker, using keep_unmanaged: yes is almost impossible, altough this is a desireable feature. I would like to propose adding a regex-based exclusion filter that is applied to the raw iptables rules as they are read. This regex could default to "" and therefore not exclude any rules and otherwise be supplied by the user.