Add support for line numbers in Markdown code fences

[coreyasmith]

Older versions of Ghost (2 and earlier?) used to support line numbers in Markdown code fences by adding line-numbers after the language:

```ruby line-numbers
foo = "bar"
```

This would be rendered as:

<pre>
  <code class="line-numbers language-ruby">
  foo = "bar"
  </code>
</pre>

The Prism Line Numbers plugin would then render line numbers for that code block.

This pull request adds in a markdown-it plugin that I implemented called markdown-it-fence-line-numbers to restore this functionality to Ghost's Markdown cards.