Open the libconfig file specified by sys.argv[1] and pretty-print it

def main():
    '''Open the libconfig file specified by sys.argv[1] and pretty-print it'''
    global output
    if len(sys.argv[1:]) == 1:
        with io.open(sys.argv[1], 'r', encoding='utf-8') as f:
            output = load(f)
    else:
        output = load(sys.stdin)

    dump(output, sys.stdout)