mdwn: Restore historical behaviour
The Discount package in Debian historically enabled fenced code blocks,
PHP Markdown Extra-style definition lists, and an expanded character
set for tag names. Since Discount 2.2.0 those are runtime settings, so
enable them. Unfortunately Text::Markdown::Discount doesn't yet expose
the necessary constants:
https://rt.cpan.org/Public/Bug/Display.html?id=124188
The IDANCHOR option was historically also enabled in Debian, but is not
enabled here because ikiwiki does not enable the TOC option, and
IDANCHOR does nothing without TOC.
Closes: #888055