Add setting for adding Markdown extensions

[daveaglick]

Would be cool to add Markdig extensions via a setting. Should be fairly easy - check for the setting inside the template and if it's there, iterate the values and use the class catalog to find the extension and add it.

Once done add a corresponding issue to document, including pointing to the Markdig site/code where you can see available extensions. Also document how to add via bootstrapper and template.

[daveaglick]

Ended up implementing this directly in the RenderMarkdown module with a new MarkdownExtensions metadata key that can be set at the document or context (I.e. settings) level.