Feature request: not equals/not present matcher for rules

[paddycarver]

We'd love to be able to use not equals or not present as a matcher for headers when creating rules. This would allow us to create two different branches of rules: for example, one for phish testing emails that people correctly caught, and one for actual phishing emails. The ability to branch on whether or not the phish testing header is actually there or not is useful. Ideally, we'd get a regex "not equals" match, but we could certainly make do with a regular comparison.