New Feature about image_demo.py

[JiayuXu0]

For image_demo.py, I want to add these features.

1. Add the inference test and result output of the images folder, cameras, video streams, and asynchronous stream.
2. Add the switch of whether to save the result file
3. Add the switch of whether to save the inference result txt
4. Increase the adjustment of the iou threshold
5. Add a switch for whether to display the test results on the screen.

[RangeKing]

I think using glob syntax, like 'path/*.jpg', as an input source is very convenient.

[JiayuXu0]

Thanks for your suggestion.

[JiayuXu0]

For input like camera and video streams， it's better inference in serial. So maybe creating a new async_demo.py file is a better choice for the asynchronous stream.