Can be tested using existing json files eg:

    import random
    for source in json_in["sources"]:
        time = round(random.uniform(30, 600), 2)
        print("Finding nearest keyframe to {} in {}".format(
            time, source["filepath"]))
        print("Nearest keyframe: {}-{}".format(
            time, nearest_keyframe(source["filepath"], time)))

    import sys
    sys.exit(0)

TODO: seek to nearest IFrame

usage: jsonclips.py clips.json

JSON file can specify:
    - output filename
    - source filepath
    - start time
    - duration
    - transition type [default: luma]
    - transition duration [default: 30]