mdbook already does include processing of its own, and the custom
processing code has always admitted as much. we don't need it for
the mdbook build at this point if we run our preprocessors in the
right order, and maybe we can even have mdbook to return complete
pages to us that we only have to pass to lowdown without any more
preprocessing of our own.
Change-Id: Icd978acbc3b1e215fee8f062c53ab2cb2a222ab1
eldritch horrors
07ef9f9f5d