Days to keep history on a per-bucket basis

[eharris]

I like to keep my POPfile history for a fairly long period of time (7 days), but I have no need or desire to keep messages from my SPAM bucket for anywhere near that long. And if I'm neglectful and miss an [unclassified] email, I'd like to keep it even longer. I'd like to see the "Number of days to keep history" have a per-bucket override, so that I can, for example, set "unclassified" messages to stick around for a month and SPAM to stick around for only a day or two.