Add blackfriday, lunamark, maruku and rdiscount.

All but Lunamark ~ 500 stars. Lunamark is significant because it is the main Lua implementation and it is made by MacFarlene.

Missing now only stuff I didn't have the patience to install because no CLI interface e.g.:

- https://github.com/s9e/TextFormatter: PHP
- https://github.com/michelf/php-markdown: PHP
- https://code.google.com/p/pagedown/: Node, used by Stack Overflow, Showdown based.