Allow creating custom directives in FILE_CONFIG

[joseluis]

A syntax needs to be defined. E.g.:

directive : mydirective : <code>

directive : li_subdir : for F in $( \
        find ../[0-9]* -maxdepth 0 -type d \
        | sed 's/^\.\.\///'); do \
    echo "<li><a href=\"$F\">$F</a></li>"; \
    done