Better design for content.split_header so we can have header and body

Actually the following is working at the moment:

```
function content.split_header (document_text)
local scl_text, body = document_text:match("---\n(.*)\n...\n(.*)")
log.debug(body)
local header = scl.to_table(scl_text)
return header, body
end
```

But I still do think this should have a nicer way to maintain this method in design.