toCategories method undefined

Hi Silvain, 

thank you so much for providing this plugin. While skimming through your readme, I stumbled upon the `toCategories()` method you use three times. However, I do not see where this method is defined? It does not seem to be implemented in your plugin, nor is it a standard Kirby method. What is this method about and does it work with `toStructure()` instead?

Best
Benedict