Add support for fenced code blocks

[stianjensen]

Today I tried to use fenced code blocks syntax, believing we had already enabled support for it.
I think we should enable this extension:

https://pythonhosted.org/Markdown/extensions/fenced_code_blocks.html

We should also see if we could fix the syntax highlighting in Codemirror to reflect this change.
It will not currently highlight

```
code
```

as code