Return relpath associated with the item.

def relpath(dataset_uri, item_identifier):
    """Return relpath associated with the item.
    """
    dataset = dtoolcore.DataSet.from_uri(dataset_uri)
    try:
        props = dataset.item_properties(item_identifier)
    except KeyError:
        click.secho(
            "No such item in dataset: {}".format(item_identifier),
            fg="red",
            err=True
        )
        sys.exit(21)
    click.secho(props["relpath"])