Encode time delta instead of absolute times

The current file format use absolute time spent since the beginning of the recording.
This means manually editing a file (adding / removing entries) is tricky since it requires to offset the entire remaining of the file.
Storing delta times since last command would solve this problem. It may result in slightly more error accumulation caused by rounding, but no one probably cares about that ~~(or it could be made an option?)~~.