This pull request adds the ability for the markdown renderer and the main README to recognize the other possible markdown file extension of ".mkd". And the possiblity of using other renderable file types as the readme.

I am currently working on adding reStructuredText and Emacs Org Mode file support to the file renderer.

This pull request also fixes the naming of one of the programming languages recognized from "Common" to "Emacs" lisp since they are somewhat different and should be recognized as different source types.