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.
- 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>