Markdown: simplify table processing

Overriding Parsedown's element() method which gives access to all
marked elements makes it easy to change the properties/attributes of
an element just before it is converted into HTML markup.

This addresses steps 2 and 3 (cleanup tables and tests) of issue 0030919.

Merge PR https://github.com/mantisbt/mantisbt/pull/1848

Co-authored-by: grummbeer kde@streber24.de
Co-authored-by: Damien Regad dregad@mantisbt.org