pure-pandoc: Add pandoc-crossref support

[agraef]

Currently pure-pandoc uses plain (Pandoc) Markdown to do all the cross-referencing, this might become easier and also give much nicer Markdown output (at least for in-document references) if we add support for the pandoc-crossref filter (https://github.com/lierdakil/pandoc-crossref). The downside is that the generated Markdown then needs pandoc and pandoc-crossref to be rendered correctly. OTOH, we're already using various Pandoc Markdown features which neither Bitbucket nor Github will render nicely, such as the line blocks (|) and the {.sourceCode .language} markup in fenced code blocks.

(Originally submitted 2017-03-07 as BB #47.)