[slides] support role attribute in listing template

Author: rmannibucau

slides-core/src/main/resources/slides-core/slides/template.bespoke/listing.html.slim

@@ -0,0 +1,16 @@
+/ forked to handle classes for sources
+- _has_title = title?
+- content_for :pre
+  - if style == 'source' && (_lang = local_attr :language)
+    / NOTE class attribute required for highlight.js and prettify
+    / QUESTION should we add a "highlight" class to pre?
+    pre.source id=(id unless _has_title) class=role : code data-lang=_lang class="language-#{_lang} prettyprint" =content
+  - else
+    / QUESTION should we add a class to pre?
+    pre id=(id unless _has_title) class=role : code =content
+- if _has_title
+  figure.listing id=id
+    figcaption=title
+    - yield_content :pre
+- else
+  - yield_content :pre