Add support for non-feed podcast sources

[elharo]

E.g.

$ podcast-archiver -f https://podcasts.apple.com/us/podcast/kaleidocast/id1035427016

Downloading archive for: Untitled Podcast


✔ Archived all episodes.

Done.

In fact this page is not an RSS feed but an HTML page. There should instead be some sort of error message indicating this.

Even better, the archiver could recognize these URLs and find the right RSS feed.