Decrease default score filter to 0.6

Given that custom-trained Detecto models often output predictions with scores of 0.6 or 0.7, the default score_filter parameter for visualize_detect_video and visualize.plot_prediction_grid has been lowered to 0.6.

To prevent this change in behavior, you can explicitly set score_filter=0.8 as follows:

from detecto import visualize

visualize.detect_video(model, 'input_vid.mp4', 'output_vid.avi', score_filter=0.8)
visualize.plot_prediction_grid(model, images, score_filter=0.8)