docs: function cx(classNames) {
    var classNamesArray;
    if (typeof classNames == 'object') {
      classNamesArray = Object.keys(classNames).filter(function(className) {
        return classNames[className];
      });
    } else {
      classNamesArray = Array.prototype.slice.call(arguments);
    }
  
    return classNamesArray.map(getClassName).join(' ');
  }