function classNames(...classes) {
    let classNames = [];

    classes.forEach((c) => {
        if (typeof c === 'object') {
            for (let cName in c) {
                if (c[cName]) {
                    classNames.push(cName);
                }
            }
        } else if (c) {
            classNames.push(c);
        }
    });
    return classNames.join(' ');
}
Class names' handler, inspired by https://github.com/JedWatson/classnames but very simplified
@param {String|Object} classes - class names, could contain strings or object