processing: Add ability to specify processors per mount. The user can now specify which processors to use for each mount (i.e. each assets directory). This is mostly for disabling anything but `copy` for websites using Grunt or similar asset pipelines instead of the built-in one.

#!/bin/sh

CUR_DIR="$( cd "$( dirname "$0" )" && pwd )"
CHEF=${CUR_DIR}/../bin/chef
OUT_DIR=${CUR_DIR}/../piecrust/resources/messages
ROOT_DIR=${CUR_DIR}/messages

$CHEF --root=$ROOT_DIR bake -o $OUT_DIR
rm ${OUT_DIR}/index.html