Filter: use Pandoc template in pre-processing for Hugo

[cagix]

When pre-processing Markdown files with Pandoc to prepare for website generation with Hugo, use custom Markdown templates for Pandoc. This will replace some day our custom archetype definitions for Hugo ...

This could be something like

---
$if(archetype=="home")$
archetype: home
$end$

title: $title$
...
---

$if(archetype=="lecture-cg")$
${ lecture-cg.md() }
$else-if(archetype=="lecture-bc")$
${ lecture-cg.md() }
$else$
${ lecture.md() }
$endif$

$-- lecture-cg.md

$if(titleblock)$
$titleblock$
$endif$

${ tldr.md() }
${ outcomes.md() }

$body$

${ quizzes.md() }
${ challenges.md() }
${ bib.md() }

related to #52 and #30