Allow for multi-line description in blog posts

[rku]

With current code, one can only supply a single line for the 'Description:' header in blog posts. This works, but causes noticeable pain when writing summaries of longer than 70 characters.

[horazont]

This is more of an issue in the markdown plugin. We could have a look if we can somehow work around on that limitation.

edit: and by plugin, i actually mean the markdown2 package