Fetch chapter names [if available] for transcript segments

[t0r0id]

This can be useful to chunk transcripts into chapter level documents for RAG.

[humpydonkey]

Hi @jdepoix ,
do you know if someone is actively reviewing this PR?
i think it'd be a really useful feature for a number of users of this library.
would appreciate the team to review and support this feature soon!
(i had to fork the repo and apply this change in the meantime.)