Enhanced metadata: Scene detection

[Julusian]

Depends on #16 #14

Generating scene detection information (disabled by default) for media files, would be a useful extension of #16 and #14
The result could be included as a list of timestamps in the json returned

https://stackoverflow.com/questions/35675529/using-ffmpeg-how-to-do-a-scene-change-detection-with-timecode