add support for tracks.json

[jpjagt]

I encountered an encoding issue when providing a tracklist with Japanese characters. I worked around this by adding support for providing a json file with tracks.

This code automatically detects if a tracks.json output file is manually supplied by the user, and if so, takes the title & timestamps from this json object.

Right now, the json object should have one attribute, tracks, which maps to a list of dicts with title and start_time. Maybe it could just be the list itself, the tracks wrapping is not really necessary...

Todo: add documentation in the readme.