[xmonad] darcs patch: H.DynamicLog: Add format strippers for xmobar and dzen

As shepheb plans to feature urgency hooks in an upcoming 'Pimp your XMonad', it seemed time to submit these, although I can't shake the feeling that there should be a better way to write them, even sticking to base libs. They allow removing ppHidden formatting to independently format ppUrgent. I'm reasonably sure they're correct, at least passed all my test cases. Many thanks to b4taylor for writing the first iteration of stripDzen, showing me how to use span properly. (p.s. this patch is huge compared to others even though it's perhaps only 30 lines changed. Is that because I attached manually rather than just darcs send-ing?) regards, -- wmw