Perhaps the OP really is talking about the Messages.app UI, but the screenshot on github imply that this is some sort of alternate UI (curses-based). Perhaps I'm misundestanding.
There is no API for Message; this github repo was scraping/scripting the Message.app GUI to create a Message command line client. OSX has removed the ability to scrape/script Messages.app GUI, so the github repo is dead in the water.
Perhaps the OP really is talking about the Messages.app UI, but the screenshot on github imply that this is some sort of alternate UI (curses-based). Perhaps I'm misundestanding.