sources: Make the `SimplePageSource` more extensible, fix bugs in `prose` source. The `SimplePageSource` now calls a `_populateMetadata` function that subclasses can override to add/edit their custom metadata everwhere it would be returned to the system.

#!/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