Better handling of "Re:" and "Fwd:" in subjects

[dbarnett]

When matching on the subject, I found that "Re:" and "Fwd:" prefixes in the subject can mess up the match for a rule that would otherwise match. For example: Rule: (subject /^Invitation:/) Subject: "Re: Invitation: Blah"

The explicitness in itself isn't necessarily good or bad, just something to be aware of, but when combined with ignoring old messages on a thread it gets especially frustrating: it'll ignore the initial "Invitation: Blah" subject that would have matched and then complain about the later "Re:" subject that doesn't. Maybe that could be tweaked to make the matching a little more forgiving?