Parse command line arguments.

def _cmdline(argv=None):
    """ Parse command line arguments.

    """
    parser = ArgumentParser()
    parser.add_argument("-c", "--config", action="append",
            help="config file [etc/config.yml]")
    parser.add_argument("-v", "--version", action="version",
            version="pipsort {:s}".format(__version__),
            help="print version and exit")
    parser.add_argument("-w", "--warn", default="WARNING",
            help="logger warning level [WARNING]")
    parser.add_argument("search_term", type=str,
                        help="Search term for PyPI")
    args = parser.parse_args(argv)
    if not args.config:
        args.config = ["etc/config.yml"]
    return args