Evo: add output to a json file option when running on a video

[tdurand]

Based on the detector.c code that does the same for a single image

This add an option when running on a video to output all results in a JSON file

./darknet detector demo cfg/coco.data cfg/yolov3.cfg yolov3.weights file.mp4 -dont_show -json_file_output result.json

Not sure if you want to merge this, but at least I have it shared with people that would want such feature

I updated the README also

[haviduck]

This works as described and have helped tremendously in my work, the stored json has all detected objects. but somehow neither my console or my preview window show objects. Is this just me messing up the build?

[tdurand]

Yes, you should uncomment the line:

//draw_detections_cv_v3(show_img, local_dets, local_nboxes, demo_thresh, demo_names, demo_alphabet, demo_classes, demo_ext_output);

And run the command without the flag -dontshow

[cenit]

looks like i don't have permission to update PR to latest master.
opendatacam#22 should do the trick