#207 utils: detect soundcloud playlists rss feeds

Only soundcloud user feeds were supported until now.

This adds playlist rss feed support and closes #207 (closed).

TODO:

• basic implementation

• test if there is a user page that also contains a playlist id

  maybe check for https://soundcloud.com/[a-z\\-]/sets/[a-z\\-]

• add test case

• cargo fmt, rebase