Fenced code blocks require blank line above; not required in GFM

[tomhundt]

In order to get a fenced code block to be recognized by Markdown Extended, you must leave a blank line before the opening marker, like this:

some text

```
x = 5   # some code
```

The blank line is not actually required by GFM. GFM actual spec. Compare to e.g., the MarkdownEditing > Markdown GFM Sublime syntax.

(On the plus side, the backticks are actually recognized as a separate scope available to be styled. Yay!)