new feature: list good but dormant feeds

[tsrodm]

Empirically, I've found a fair number of feeds in my podcasts.conf
that are valid, but stale; ie the website is no longer providing any
new podcasts.

How about a logging feature that would list URLs for these
sites. Maybe a switch:

--dormant MONTHS

such that above switch would create a STDERR warning with text something like:

"last update for URLHERE was DATEHERE"

so we could grep the logs, and remove those sites from podcasts.conf.

[zerodogg]

That's not a bad idea. I'm in the process of converting the log format for goldenpod to JSON (https://github.com/zerodogg/goldenpod/tree/jsonlog). It won't take much to add some timestamps to it.