[Feature Request] Add podcast subscription by RSS

[lamons]

I know Library editing is not the main purpose of SelfPlayer but I think a functionality of adding RSS subscription would be really useful.

• Most metadata could be fetched directly from the feed so no complex additional edit UI is needed.
• It's quite often (at least in my use case) I want to subscribe to a new podcast while not on desktop, it would be convenient to be able to do that in ShelfPlayer alone.

[rasmuslos]

I thought about doing this in the past, but ultimately decided against it, because it would require podcast management features to download the existing episodes and setup a schedule afterwards. But I am not opposed to the idea.