add Yavli

Fuck Yavli.

They've got some of the absolute worst "promoted content" I've ever
seen, and they try to be sneaky about it, too -- they've got a huge
stable of domains that they use to evade filters.

I don't like CSS rules, but every instance of Yavli I've seen uses this
same CSS class -- it's certainly a lot easier, and more effective, than
trying to play whack-a-mole with every one of their domains.