pandoc

convert all the (text) things, check out pandoc

convert markdown to html

pandoc README.md -o README.html

convert html to markdown

pandoc README.html -o README.md

convert anything to markdown

also extract attachments

pandoc input.epub -t gfm --extract-media=./images -o output.md

drop html tags which cannot be converted (to markdown)

pandoc input.epub -t gfm-raw_html --extract-media=./images -o output.md