discount support

mdwn: Can use the discount markdown library, via the
Text::Markdown::Discount perl module.

This is preferred if available since it's the fastest currently supported
markdown library, speeding up markdown rendering by a factor of 40.

That is to say, when only rendering a lot of markdown, discount is 40x
faster. When building a ikiwiki site, ikiwiki's other overhead gets in the
way, but I still see significant speedups. Building the ikiwiki docwiki
dropped from 62 to 45 seconds, for example.

However, when multimarkdown is enabled, Text::Markdown::Multimarkdown is
still used.

While discount contains some nonstandard markdown extensions,
including tables and footnotes, AFAICS most of them are not
enabled by default in the perl bindings.

I consider sticking to non-extended markdown a desirable thing, since this
is probably not the last markdown engine. In particular, sundown is waiting
in the wings to get packaged and get a perl binding.

----

Reviewing all the showdown extensions, here are the ones that are enabled:

centered paragraphs:

->centered<-

image sizes: [dust mite](http://dust.mite =150x150)

<style>..</style> blocks are eaten. The perl binding does not provide
access to the gathered CSS. This is not legal html anyway, so unlikely
to cause breakage.