Use github-style code highlighting

From: https://github.com/aahan/pygments-github-style/blob/master/github.css
Merge pull request #10 from alanorth/pygments-style

Use github-style code highlighting
Merge branch 'master' of github.com:nairobilug/pelican-crowsfoot

* 'master' of github.com:nairobilug/pelican-crowsfoot:
  Use github-style code highlighting