This adds smu (simple markup).
I've been using this in production for a while.

Does it belong in markdown.scm? Technicaly it is not markdown,
but it's the closest I could find when I moved it from my
package repo to the guix repo.
-- 
ng0
OpenPG: A88C8ADD129828D7EAC02E52E22F9BBFEE348588