Automatically delete old entries

[zaknesler]

Yarr does this, though I never experienced it.

I'm guessing once a feed reaches X items, anything older should be deleted. This should be overridden if the entry is saved. I'd also like to explore a "priority" assignment to a feed to only delete low-prio feeds if they have been defined. Along with (or at the very least) the ability to disable pruning (yeah let's call this feature pruning) entire feeds.

All of this should be user-configurable and not global.