mdwn: Automatically detect which libdiscount flags to use

mdwn: Automatically detect which libdiscount flags to use

Unconditionally passing arbitrary numbers as flags turns out to be a
bad idea, because some of the "unused" values have historically had
side-effects internal to libdiscount. Detect whether the known flags
work by rendering short Markdown snippets the first time we htmlize,
checking whether each known flag is both necessary and sufficient.

Signed-off-by: Simon McVittie <smcv@debian.org>