Blacklist mode

[ShadowJonathan]

I came across this in https://github.com/ruma/ruma/pull/1161; It'd be useful for ammonia to have a blacklist mode, instead of a whitelist mode, to remove specific tags from HTML, instead of retaining specific tag.

The usecase here is removing unwanted inserted tags from an otherwise wholly trusted and/or intended-to-be-preserved HTML snippet.