Problem with embedding custom css

I want to embed custom css into the generated HTML.

When using this
```
"pandoc.htmlOptString": "--standalone --embed-resources --to=html5 --css=styles/pandoc_markdown.css"
```
the css file is still linked and not embedded:
```
  <link rel="stylesheet" href="styles/pandoc_markdown.css" />
```

Is this expected behaviour or am I doing something wrong?

Thanks for the great extension!