Program entry point

def main():
    """Program entry point"""
    parser = argparse.ArgumentParser()
    parser.add_argument('input',
                        metavar='INPUT',
                        help='Input directory with PNG files')
    parser.add_argument('output', nargs='?',
                        metavar='OUTPUT',
                        help='Output directory')
    parser.add_argument('--apikey',
                        metavar='APIKEY',
                        default=find_apikey(),
                        help='TinyPNG API Key')

    args = parser.parse_args()
    _main(args)