Server safe mode

Adding the SERVER safe mode level turned out to be a rather minor change and seems to be a good fit.

The header_footer default setting was a bit wider of a change, but again, it seems to make the most sense.

The reason I think the header_footer default is important is because we don't want people switching from Markdown to AsciiDoc and be totally taken by surprise when they start getting a full HTML document. On the other hand, if you are rendering to a file (or using the cli), then it makes a lot of sense to write a standalone document. To me, it just feels natural now.