Added a rescan hook.

Added a rescan hook.

This is needed for the po plugin vs. e.g. meta titles.

In order to get rid of the ugly "rebuilding all pages to fix meta titles" thing,
Joey suggested to make "po, at scan time, re-run the scan hooks, passing them
modified content (either converted from po to mdwn or with the escaped stuff
cheaply de-escaped)". This would unfortunately not work, as the meta plugin
gathers its data using the preprocess hook in scan mode: it would overwrite with
buggy data the correct data we would have forced it to gather in po's scan hook.

We then need a hook that runs *after* the preprocess hook has been run in scan
mode, but *before* any page rendering is started. Hence this one.