Improve rendering of code blocks / code listings

[skaldarnar]

We are a technical project, and we'll probably show source code and code listings on our website every now and then.
Thus, it would be great if such code listings would fit into the overall theme and look better.

Right now, code blocks are hard to read due to a small font size.

I think a layout similar to how this is rendered here on Github would be a good improvement. Ideally, we'd also support language tags to get code highlighting.

[Rukki13]

Hi, i've fixed the font size and imported the prism CSS style sheet.
it now looks better:

For better syntax highlighting tailored for a specific language, like in the example below, the language must be specified in the md file above the code blocks.

[Rukki13]

I've made a pull request here: #243