Fix issue #5877 with tests

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 added in `test_strings.py`

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