Expand HtmlTemplate's Markdown Function

[billstclair]

Defining pages with JSON is already getting very old. WIth a little more work, a can add to the "#md" macro links, images, and lists. That will be enough to convert the bulk of the site as it is now to plain Markdown.

Then add automatic determination of whole page format. This could use the file suffix, but I don't want the address bar to show xosbow.com/about.md, so it needs to be determined by the content of the page. Maybe if it starts with "{" or "[", its JSON, and otherwise it's Markdown.

[billstclair]

Also, switch to using the HtmlTemplate.PlayDiv module instead of running a copy of that code in Xossbow.elm.