v0.13.1 – add some improvements for modern feed readers

[Moonbase59]

Part of this code will be important if ever we tackle AzuraCast Podcast RSS feeds. But Podcasts are already handled natively by AzuraCast.

We mainly send some more metadata so a modern feed reader knows

• which old (previously downloaded) items (programmes) in the feed can be removed,
• at what times the linked streams are valid and actually play what is advertised.

This improves user experience, since

• less items are shown in a feed (depends on how many days into the future you let azuracast_xmltv generate),
• the player knows (in theory) if it should play a stream. It makes no sense trying to play a programme from the past or in the future—the AzuraCast stream will always only play what’s on now.

Update optional. You can if you want less items in your feed, but it’s not a required update.