Update _config.yml

If you want markdown rendering to be the way it is in GitHub wiki and project pages (extended markup syntax including fenced code blocks!) then you have to use redcarpet processor.
Adding syntax highlight css required by pygments
Wiring-up syntax.css into default layout.
Fixing: we'd only want auto overflow for x axis.
Merge pull request #40 from inadarei/master

Github Markdown support including code color highlighting