Improve custom chapters

[Chargothrond]

Currently, one has to define them manually with a Python dictionary.

There are definitely better and more user-friendly approaches, but it is a low priority for me currently.

If there are chapters, clean_song_titles and edit_times work better for me so far for all my examples.

It would however be useful for cases where there are no chapters, but then one might also consider parsing the information (if available) from the description, which comes with its own challenges.