Support Markdown Fenced Code Block Grammar

[optikfluffel]

I'd love to have syntax highlighting inside markdown code blocks:

```elixir
IO.puts("some output")
```

There is an Example Repo I found in microsoft/vscode#34891, but I just started using VSCode and don't feel confident enough (yet) to open up a PR and give it a try on my own.

[mtarnovan]

Has this been fixed? I still don't see syntax highlight in Elixir code blocks in Markdown files. I took a brief look at the commit and I'm not sure if the plantuml references are accidental.