function load_extensions_from_config(section) {
        section.loaded.then(function () {
            if (section.data.load_extensions) {
                var nbextension_paths = Object.getOwnPropertyNames(
                    section.data.load_extensions);
                load_extensions.apply(this, nbextension_paths);
            }
        });
    }
Wait for a config section to load, and then load the extensions specified
in a 'load_extensions' key inside it.