(markdown) snake_case term causes italics in rest of file

**Describe the issue**

If you have an unescaped term in snake_case in a Markdown file, it puts highlight.js into "italic mode" when it should not. GitHub-Flavored Markdown (GFM) and others seem to not italicize underscores in the middle of a word.

**Which language seems to have the issue?**

- markdown

**Are you using `highlight` or `highlightAuto`?**

- `highlight`

**Sample Code to Reproduce**

https://jsfiddle.net/mn15jwg7/

```
Header_under

code listing; italic

More text, still italic.
```

Here's how highlight.js renders it on the playground. Note how the `_` puts it into Italic mode and it never leaves, even in subsequent blocks:

Here's how GitHub renders the same markdown: