Parse command line arguments.

    Returns:
        options, args

def parse_options():
    """
    Parse command line arguments.

    Returns:
        options, args

    """
    parser = argparse.ArgumentParser(description='Video downloader by radzak.',
                                     prog='RTVdownloader')
    urls_group = parser.add_mutually_exclusive_group(required=True)
    urls_group.add_argument('urls',
                            type=str,
                            metavar='URL',
                            default=[],
                            nargs='*',
                            help='urls of sites containing videos you wish to download'
                            )

    urls_group.add_argument('-f',
                            type=argparse.FileType('r'),
                            dest='files',
                            metavar='FILE',
                            default=[],
                            nargs='*',
                            help='text file with urls of sites containing videos you '
                                 'wish to download '
                            )

    urls_group.add_argument('-o',
                            type=str,
                            dest='onetabs',
                            metavar='ONETAB',
                            default=[],
                            nargs='*',
                            help='onetab links containing urls of the videos you wish to download'
                            )

    options = DEFAULT_OPTIONS

    # TODO: add dir option that defaults to the DEFAULT_OPTIONS['dl_path']

    args = parser.parse_args()
    return options, args