filters: add lowdown filter for readme

it's suitable only for converting markdown to html, but i don't need
more. lowdown is better than about-formatting.sh because it doesn't
require shell. as i run this server on openbsd, web server is chrooted
so every binary should be copied to chroot directory. since chroot is
considered a security measure to restrict shell access, copying shell
executable to chroot directory would be stupid.