Highlight Rel snippets inside Markdown

[nchammas]

The extension works and I see syntax highlighting for .rel files. However, I don't see highlighting for rel code snippets within a Markdown document.

<!-- file.md -->
# Some Heading

Look at this code:

```rel
def output = data
```

I assumed that any extension installed for a given language would automatically be used to highlight Markdown code snippets, but perhaps that's not the case.

Is this a gap in the extension, or something VS Code itself isn't doing correctly?