Improve traversal

[matteocontrini]

After #3 was merged, some issues were introduced:

• empty bullet points are rendered in lists
• empty headings are rendered
• headings "symbols" (+++ or ---) now have a fixed length instead of depending on the heading length

A possible solution is to restore the previous nested traversal with multiple levels of HtmlTraversal instances, and then passing around a LinksTracker object to gather information about links that should be rendered only at the end of the top-level traversal instance.