Add support for GitHub-flavoured markdown code fences for highlighting

Add support for GitHub-flavoured markdown code fences for highlighting

This commit adds a new PygmentsCodeFences config option (default false), which if true will allow GitHub style backtick code fences around code, which will then be rendered by Pygments.

For example:

``` language
your code
```

can be used instead of {{< highlight language >}}your code {{< /highlight >}}.

Fixes #362