Suggestion: YouTube-DL for reliable caption retrieval

[YouTube-DL](https://github.com/ytdl-org/youtube-dl) has a [number of options](https://github.com/ytdl-org/youtube-dl#subtitle-options) for downloading and retrieving subtitle URLs. Using it would be a good way to provide reliable retrieval, and future-proof this project for any changes YouTube makes to their caption system.