add layout: plain-markdown

useful for postprocessing with tools like pandoc (read: simple PDF output).
more info/usage in the sub-README.

examples:
- [docco.pdf](https://gist.github.com/eins78/5293300/raw/docco.pdf)
- [backbone.pdf](https://gist.github.com/eins78/5293300/raw/backbone.pdf)
- coffeescript's [scope.js](https://gist.github.com/eins78/5293300/raw/scope.pdf)
- [underscore.pdf](https://gist.github.com/eins78/5293300/raw/underscore.pdf)

bonus:
- [underscore.epub](https://gist.github.com/eins78/5293300/raw/underscore.epub)
- [underscore.latex.txt](https://gist.github.com/eins78/5293300/raw/underscore.latex.txt)
- [underscore.asciidoc.txt](https://gist.github.com/eins78/5293300/raw/underscore.asciidoc.txt)
- [underscore.mediawiki.txt](https://gist.github.com/eins78/5293300/raw/underscore.mediawiki.txt)