Allow caller to specify language when rendering code-snippet from handlebars template

[samselikoff]

I have a HBS template and I want to show a diff, which I would normally use ```diff for (github fences). Here's my template:

{{#docs-snippet name="your-snippet-name"}}
  model() {
-   return this.get('store').findAll('post');
+   return this.get('storefront').findAll('post');
  }

  model() {
-   return this.get('store').findRecord('post', 1);
+   return this.get('storefront').findRecord('post', 1);
  }
{{/docs-snippet}}

This works but I want to be able to say, "this snippet is actually a diff snippet (or a ruby snippet, etc.)". Perhaps a lang="diff" option to the {{docs-snippet}} component.