Added extensive transformers via :custom-markdown-transformers config key

[emidln]

For one reason or another, I needed to customize the markdown being generated. markdown-clj helpfully provides a way to customize this behavior, and we even use it, but we don't expose it in any way.

I'm not sure what the best way of loading misc functions happens to be (i.e. should I create another registry of stuff?), so the implementation is very much "Suggested Improvements Welcomed". That said, this does work.

I chose to have a key in the config.edn that is a keyword referencing some var that can contain a list of transformers. A snippet from my config.edn would look like this:

:custom-markdown-transformers [:cryogen.ddftwiki-markdown/custom-transformers]

I obviously created a src/cryogen/ddftwiki_markdown.clj with a (def custom-transformers [....]) in it from my template.

I'm unsure how many people would be interested in this, and if it's just me, I'm happy to sit on my own fork forever. That said, it does work for me.

[lacarmen]

I think the implementation makes sense. Would you mind updating the config schema in cryogen-core and adding a new page to the docs to reflect this?