Catch-up on lists

**Is your feature request related to a problem? Please describe.**
I got a science list, I got a cartoons list, I got a friends list. I read them all entirely. The rest go in the 'other' list which I skim at best. If I use catch-up then it's bundling these all together meaning all the stuff I already read in Science and Friends are in the catch-up list despite having already been read.

**Describe the solution you'd like**
Catch-up function can have a drop-menu to include "home" as it presumably is doing now, but also each of the lists.

Might be nice if it also had the Federated and Notifications and stuff I guess? If that's easy. Doubt I'd use that though. Really popular people might wanna do catch-up on their notifications I suppose.

**Thanks**

Loving Phanpy in general. Easiest skim of any of the readers I've used even before that catch-up function. Thanks.

Start by locating the catch-up implementation and the existing home-feed selection in the Phanpy client. Trace how Mastodon lists are loaded and selected, then define completion as allowing catch-up to target an individual list without including items already read in other lists; federated and notification feeds are optional and unspecified.

Written by the indexing model from the issue text.