Markdown support

It would be great to have markdown support for text entry. Why?

• Markdown is much more widely known and supported than the custom crabgrass text format
• It would allow much easier interoperability between crabgrass and other systems (e.g. exporting markdown from a crabgrass wiki to include it in a blog post or public wiki)

I'd propose to add markdown as an additionally supported format and have a select box on the form to set the text format in the main body field.