Threshold option during detection

[SlavaKeshkov]

Hey @zzh8829, thanks for your code, works great. I was thinking it would be a good idea to have a detection threshold flag --thresh for the detection.py or conversion.py. Do I get it right that iou_threshold and score_threshold in lines 190-192 of models.py is the only way to change threshold for prediction? Let me know, this flag along with num_classes flag would be a great boost to the repo.

[zzh8829]

done