Allow hiding of individual items from feeds

Firstly, thank you for such a well-made piece of Free Software. I'm really glad it exists, and I want to use it a lot, but unfortunately it doesn't seem to keep track of which items I've already seen, which makes the interface a bit overwhelming.

I had assumed that showing the read/unread status of each item would be a standard feature for a feed reader, so I searched through the closed issues and found the feature request in issue #121 (closed), which I agree with. I also see that you gave some very reasonable justifications for why that request couldn't be implemented.

Instead, then, I'd like to offer a design which might satisfy some users, while also respecting your objections. My main idea is that users should be able to opt in (per feed) to having the software keep track of which items are hidden, so this new functionality wouldn't be the default. When the user selects this option (at feed creation time), there can be a warning displayed that this has a performance penalty, and it could also warn them that more data about them would be stored.

Importantly, though, I want to point out that this feature should allow users to hide items from the feed without actually visiting the page that the item links to. Instead, there should be an icon next to each item (perhaps appearing on-hover) which allows the user to hide it, equivalent to "Mark as read" in an email client.

I'm imagining that the users who use this feature would aim to have no items showing in any of the feeds (equivalent to the "inbox zero method", to use another email client metaphor), so most of the time there wouldn't actually be any private or personal information stored in the database for such users.

Finally, I think it would be helpful to also add a button to show all hidden items, in case a user hides an item by mistake and wants to unhide it again. Similarly there should be a "Hide all" button, to save time if the user doesn't want to click the individual hide buttons for every item.

Thank you again for what you've created so far, and I hope that my suggestion is practical enough to be implemented.