Fix markdown2asciidoc function for pandoc >= 3.0 (closes #2017)

[thomasjm]

Pass different arguments to pandoc depending on whether the version is >= 3.0, to fix #2017.

[takluyver]

The fix looks good to me, but I think the asciidoc tests should be extended to check for Markdown cells. The notebook used in test_export already has Markdown cells, but the test still passes if they're missing from the output.

(I'm also kind of surprised that nbconvert carries on if pandoc fails, but that's a bigger question)

[thomasjm]

Okay, added a test. I manually confirmed that the new test fails without the fix in this branch.

[takluyver]

Thanks!