Fix a bug on Windows when saving an output img.

[liuslnlp]

The separator in Windows is "\" and in Linux is "/", so this line will be invalid on Win system

det_names = pd.Series(imlist).apply(lambda x: "{}/det_{}".format(args.det,x.split("/")[-1]))

and I modified it to

if platform.system() == 'Windows':
        det_names = pd.Series(imlist).apply(lambda x: "{}\\det_{}".format(args.det,x.split("\\")[-1]))
    else:
        det_names = pd.Series(imlist).apply(lambda x: "{}/det_{}".format(args.det,x.split("/")[-1]))