Add ability to sort by anime that have unwatched episodes

[crxssed7]

This is most likely not be possible via the AniList API but I think it would be a nice to have anyway. In the anime listing pages, it would be nice to have a filter option that only shows anime that have unwatched episodes. Currently it shows progress in a pinkish red if I'm behind on an anime but I like having my list clean and focused.

This might be a niche feature to have so I don't mind if not planned, and to be fair would only make sense on the CURRENT list.

[lotusprey]

All collection filtering I do locally, so this is possible. But @crxssed7, maybe instead of a filter, there could be a new sorting option that orders entries by how much you have fallen behind. For example, for the first entry you have to catch up with two episodes, for the second you have to catch up with only one episode and with the rest you've caught up, so they're sorted alphabetically maybe.

[crxssed7]

@lotusprey Yeah I think this makes more sense actually instead of just hiding up to date entries.

I've not contributed in a while so I'm happy to take this on if you'd like (I might not get around to it till next week though).