RFE: store previous cursor

[kpfleming]

I just had a situation where my email delivery (not sending) was broken, so the batch of daily journal-brief emails I usually get were lost. I'd like to be able to regenerate them, but without knowing what the cursor was prior to that batch being sent there's no practical way to do it (I could probably recover the cursor files from a filesystem snapshot, but that will be more complicated).

So, I'm thinking that journal-brief itself could preserve the 'last cursor' (just one level) when it writes a new cursor.