[Podcast] Auto-download new episodes

[ghost]

Some prefer to download episodes automatically, to then have them available for future offline listening. This should be a per-podcast setting.

This goes great together with automatically add to queue/playlist at top/bottom. #462

[notnout]

I like the suggestion. To add more detail:

• The common case here is listening to podcasts on a plane. Currently it relies on me not forgetting to download couple pods ahead of time. My memory is generally a bottleneck for many things.
• Another case is users that can't afford to pay for downloading pod using their mobile data plan.