This is heavily based on https://github.com/ryantsangai/fontello-cli but with a few tweaks to let it behave as I want. Notably, editing sessions can provide a save URL which we can use to hit our local proxy server to make the config auto-download.
This also lets you save command-line arguments to a secondary config file.
$ npm install --save-dev @ffz/fontello-cli
$ ./node_modules/.bin/fontello-cli --help