Implement a Markdown Renderer

Goals:

• Ability to edit Markdown in the WYSIWYG editor
• Ability to convert any syntax into Markdown. For example convert HTML to Markdown. This would make XWiki Rendering more powerful and potentially more useful to non-XWiki users, as a library. We could write a tutorial about using the XWiki Rendering to achieve this easily.
• If we offer an integration with GitHub this allows users to continue using Markdown to view/edit their GitHub pages inside XWiki using the WYSIWYG editor and save them back.