Add Visual Studio extension for Elixir (elixir-lang#723)

I just saw that there is no extension for Visual Studio Code users.

Author: FuriKuri

_includes/code-editor-support.html

@@ -8,5 +8,6 @@ <h3 class="widget-title">Code editor support</h3>
     <li><a class="spec" href="https://github.com/SteffenBauer/elixir-gtksourceview">GtkSourceView (gedit)</a></li>
     <li><a class="spec" href="https://github.com/lucasmazza/language-elixir">Atom Package</a></li>
     <li><a class="spec" href="https://github.com/KronicDeth/intellij-elixir">IntelliJ Elixir</a></li>
+    <li><a class="spec" href="https://marketplace.visualstudio.com/items?itemName=mjmcloug.vscode-elixir">Visual Studio Elixir</a></li>
   </ul>
 </div>