sources: Add an `IListableSource` interface for sources that can be listed. Page sources that implement this interface can be "iterated through", like a hierarchy, which is useful for the upcoming `Linker` feature.

#!/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 $@