Per-feed filtering

[zaknesler]

This is something I saw suggested to add to yarr, and I think it'd be great. Some feeds produce a lot of garbage that would be nice to filter out from even being saved to the database. So a user-configurable filter per-feed would be super handy.

Functionality-wise, this is pretty complex and can be done in a bunch of ways, but some ideas for negative/positive filters might be:

• title NOT "ai"
• content HAS "fujifilm" AND "film simulation"

...or something like that