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>