Apply `processor` or `self.processor` to `item`.

def process_one(self, item:ItemBase, processor:PreProcessors=None):
        "Apply `processor` or `self.processor` to `item`."
        if processor is not None: self.processor = processor
        self.processor = listify(self.processor)
        for p in self.processor: item = p.process_one(item)
        return item