Expecting robust processing of irregular headdings

[simonmysun]

This input fails on my computer:

## Heading 2
#### Heading 4

[medfreeman]

After studying the question, it seems that the idea to use a markdown representation for these kind of broken unordered lists was a bad idea after all.
Conflicts with fenced code blocks among other things arise when having empty list levels between entries.
I'll switch back to a tree to html representation in the next version, removing the markdown callback option.
This will be a breaking change anyway, although not sure if anyone uses this feature.

[simonmysun]

Thank you very much for your effort!

The problem I met is that, I need real time rendering for changing texts. When user alter the content, the middle state could be unpredictable, which includes the problem described here.