add keybinding for "du" or "ncdu" if available

[Docbroke]

example code

        ## run du/ncdu to get sizes
        u)
            [[ -e "${list[scroll]}" ]] && {
                clear_screen
                reset_terminal
                status_line "${list[scroll]}"
                type ncdu &> /dev/null && ncdu "${list[scroll]}" || du "${list[scroll]}"
                setup_terminal
                redraw
            }

created pull request https://github.com/dylanaraps/fff/pull/183