feat: make the markdown rendered configurable in VoilaExporter

Example configuration (e.g. in voila.json) could be:

'VoilaExporter': {
    'markdown_renderer_class': 'my.package.MyMarkdownRenderer'
}

Closes #234

voila/exporter.py

@@ -33,16 +33,18 @@ def image(self, src, title, text):
 class VoilaExporter(HTMLExporter):
     """Custom HTMLExporter that inlines the images using VoilaMarkdownRenderer"""
 
+    markdown_renderer_class = traitlets.Type('mistune.Renderer').tag(config=True)
+
     # The voila exporter overrides the markdown renderer from the HTMLExporter
     # to inline images.
-
     @contextfilter
     def markdown2html(self, context, source):
         cell = context['cell']
         attachments = cell.get('attachments', {})
-        renderer = VoilaMarkdownRenderer(escape=False, attachments=attachments,
-                                         contents_manager=self.contents_manager,
-                                         anchor_link_text=self.anchor_link_text)
+        cls = self.markdown_renderer_class
+        renderer = cls(escape=False, attachments=attachments,
+                       contents_manager=self.contents_manager,
+                       anchor_link_text=self.anchor_link_text)
         return MarkdownWithMath(renderer=renderer).render(source)
 
     # The voila exporter disables the CSSHTMLHeaderPreprocessor from the HTMLExporter.
@@ -53,6 +55,9 @@ def default_config(self):
             'CSSHTMLHeaderPreprocessor': {
                 'enabled': False
             },
+            'VoilaExporter': {
+                'markdown_renderer_class': 'voila.exporter.VoilaMarkdownRenderer'
+            }
         })
         c.merge(super(VoilaExporter, self).default_config)
         return c