Add option 'class' to 'rv_code' directive.

With we can set the language explicit. E.g. set python where highlight.js
fails::

   .. rv_code::
      :class: python

      >>> from fspath import FSPath
      >>> tmp = FSPath('~/tmp')
      >>> tmp
      '/home/user/tmp'
      >>> tmp.EXISTS
      False

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>

src/sphinxjp/themes/revealjs/directives.py

@@ -172,7 +172,10 @@ class RvCodeDirective(Directive):
     required_arguments = 0
     optional_arguments = 0
     final_argument_whitespace = False
-    option_spec = {}
+    option_spec = {
+        'id': directives.unchanged,
+        'class': directives.class_option,
+    }
     node_class = rv_code
 
     def run(self):