internal: Improve how theme configuration is validated and merged. * Add default theme config up-front so it benefits from the usual validation. * Add an explicit `use_default_theme_content` setting. * Add/fix unit tests.

#!/bin/sh

CHEF_DIR=`dirname $0`
if `hash readlink 2>&-`; then
    LINKED_EXE=`readlink $0`
    if [ -n "$LINKED_EXE" ]; then
        CHEF_DIR=`dirname $LINKED_EXE`
    fi
fi
python $CHEF_DIR/../chef.py $@