GitHub markdown syntax highlighting

Somehow, I missed this and haven’t used it until yesterday – GitHub markdown syntax highlighting:

Code blocks can be taken a step further by adding syntax highlighting. In your fenced block, add an optional language identifier and we’ll run it through syntax highlighting. For example, to syntax highlight Ruby code:

```ruby
require 'redcarpet'
markdown = Redcarpet.new("Hello World!")
puts markdown.to_html
```

We use Linguist to perform language detection and syntax highlighting. You can find out which keywords are valid by perusing the languages YAML file.