I absolutely don't how the file "postfixbook/postfixbook.js" has to be structured. It's not clear to me, even when reading the README.md you proposed.
Maybe something like: " { name: '$plugin_name', path: 'plugins/$plugin_name' } " But how to continue? I mean, do I have to put this inside an array or an JSON object etc. How would that look like for multiple plugins like I have?