Escape Hugo shortcodes in code blocks

[kaushalmodi]

See:

• Discussion on Hugo discourse
• Markdown, Hugo output

Hugo will automatically render this in Markdown:

```md
{{</* figure src="/images/org-mode-unicorn-logo.png" title="Org Mode + Hugo Rock!" */>}}
```

As this in HTML:

{{< figure src="/images/org-mode-unicorn-logo.png" title="Org Mode + Hugo Rock!" >}}