All Heading Levels Converted to H2 in Asciidoc

[iyaja]

The asciidoc exporter converts all heading levels specified in markdown cells to H2.

To reproduce the issue:

1. Install nbconvert
2. Create notebook with markdown cells
3. Add markdown headers with different header levels in markdown cells
4. nbconvert --to asciidoc notebook_name.ipynb

This error can be reproduced on https://jupyter.org/try as well.

As an example, here is a notebook with multiple markdown header levels:

Here is what it is converted to in asciidoc:

== This is a H1

== This is a H2

== This is a H3

== This is a H4

== This is a H5

== This is a H6

Nbconvert version: 6.0.7

[jph00]

@willingc FYI this is also being hit by us and by @allendowney at the moment. I've tracked it down to a pandoc issue in all versions since 2.3:

Because nbconvert is passing one cell at a time to pandoc, it's triggering this renumbering.

cc @hamelsmu