Pandoc Filters for CodiMD

1. Install requirements

   $ pip3 install -r requirements.txt
   $ git submodule update --init
   

   (may require plantuml.jar in the working directory)
2. Convert Markdown file

   $ (cat <header>.yaml; ./preprocess.sh <file>.md) \
       | pandoc -F codimd.py -o <file>.pdf