Adds a mask that maps to a given folder relative to `base_path`.

def bind_path(self, name, folder):
        """Adds a mask that maps to a given folder relative to `base_path`."""
        if not len(name) or name[0] != '/' or name[-1] != '/':
            raise ValueError(
                "name must start and end with '/': {0}".format(name))
        self._folder_masks.insert(0, (name, folder))