Fix posts not sorting by posted date desc.

Apparently, the parameter we were specifying if none existed was not the correct one.

Signed-off-by: Nathan Lowrie <nate@finelineautomation.com>
Added a use_markdown preference.

If the use_markdown preference is checked, the posts will process markdown like normal and the editor will be the markdown editor.  If not checked, we will process the post like it is normal HTML.  The that case, the preview breaks occur on the first horizontal rule tag.

Signed-off-by: Nathan Lowrie <nate@finelineautomation.com>