Refresh time per feed/group

[fenuks]

I'm aware that yarr is feature complete, but I'd like to propose a feature that I think would be useful to many people.

I have many feeds, hundreds of them, some are less important/not frequently changed. For example, I use atom feed to track github releases, for yarr it is https://github.com/nkanaev/yarr/releases.atom. I don't need it to update as often as e.g. news page feed, every few days would be fine, but currently there is only global option to specify how often feeds ought to be refreshed.