Pandoc-style filters for Apex

In my quest to make Apex as complete as possible before I integrate it into Marked, I’ve added Pandoc-compatible filters, which can be written in Lua for native execution, or in any language using a Pandoc JSON pipeline. The filter system runs your code on the document after parsing and before rendering. The pipeline is: Markdown → AST → Pandoc-style JSON → your filters → JSON → HTML. That means you can transform the document in any language that speaks JSON — Ruby, Python, Lua, Node, whatever — using the s