Adds new functions to the end of the pipeline.

        Functions must accept three arguments:
        - Token: A lunr.Token object which will be updated
        - i: The index of the token in the set
        - tokens: A list of tokens representing the set

def add(self, *args):
        """Adds new functions to the end of the pipeline.

        Functions must accept three arguments:
        - Token: A lunr.Token object which will be updated
        - i: The index of the token in the set
        - tokens: A list of tokens representing the set
        """
        for fn in args:
            self.warn_if_function_not_registered(fn)
            self._stack.append(fn)