Adds the ``filter_cls`` to our registry.

def register_filter(filter_cls):
    """
    Adds the ``filter_cls`` to our registry.
    """
    if filter_cls.name is None:
        return
    elif filter_cls.name in FILTER_REGISTRY:
        raise RuntimeError(
            "Filter class already registered: {}".format(filter_cls.name))
    else:
        FILTER_REGISTRY[filter_cls.name] = filter_cls