Fix issue #5877

This pull request prevent to have lists environments inside the headings
cells
which break latex code.

- First, the function `markdown2latex` has been changed in order to
  accept a markup keyword for pandoc reader, default to `markdown` as
  before( `test_markdown.py` updated too)
  - Headings cells are treated with markup
    `markdown_strict+tex_math_dollars` to remove enhancements of pandoc
    for lists and add math support
    - Secondly, `prevent_list_blocks` filter escape characters `. - * +`
      in headings cells to prevent `enumerate` or `itemize` environments
      in latex sections

      Test files accessible at:
      <https://gist.github.com/benjaminabel/4a40d1300dcf087c8695>