Add an item to the list.

        Args
        ----
        label: str
            Description of the item.
        s: str or `~.LatexObject`
            The item itself.

def add_item(self, label, s):
        """Add an item to the list.

        Args
        ----
        label: str
            Description of the item.
        s: str or `~.LatexObject`
            The item itself.
        """
        self.append(Command('item', options=label))
        self.append(s)