Support GFM style for syntax highlighting

[tpolecat]

The recent fix for #70 helps (thanks!) but there is still a problem with the machinery that attempts to guess which highlighter to use. Can we not support GFM style where you say

```scala

or java or whatever to mark code blocks and indicate desired highlighting? My particular issue is that I have blocks for which I don't want any highlighting at all (no syntax is indicated after the ```) and they end up looking very strange.