use two step conversion; allow more customization

- load document and convert in discrete steps
- set page title using the Document#doctitle method
- only source highlight if source-highlighter is hightlightjs
- only source highlight blocks with an explicit language declared
- allow showtitle, source-highlighter and sectanchors to be disabled
- explicitly set safe mode (set to default value)
- add hljs to source blocks with no language to fix style problem