Convert postprocessordirectives into preprocessordirectives, so they are

Convert postprocessordirectives into preprocessordirectives, so they are
expanded before markdown. Consequences:

 - No need to worry about markdown messing with parameters of
   preprocessordirectives. (If you had to escape stuff in one before, you'll
   need to undo that escaping now.)
 - No need for ugly </p> hacks before inlined subpages. Instead, subpages
   are wrapped in a <div>, and this prevents markdown from touching them.
   (This can also be used to add style to subpages.)
 - rss generation is less of a hack.