Figure out how to block bombing people ignored by host

[dgw]

Splitting this out of #12 because it's not going to happen easily.

Bonus points: Handle bot.config.core.host_blocks also. There doesn't seem to be a way of getting the host of the target easily, only the calling user (via trigger.host), but this is a nice stretch goal.

[dgw]

We'll leave this be some more. It's already been nearly five years, but now there's an idea for making it easier to implement.