Whitelist allowed tags by callable

This adds the possibility to filter the tags by function, similar to the way its done for attributes.

Our use-case here is that we want to dis-allow `script` and other evil tags but don't want to allow an ever-growing whitelist. And our users often add things like `<username here>` in their texts...

I don't know if there is a better way to tell python that the allowed_tags-function is a static function.

Oh, I also renamed the tests for attributes to `bleach.clean` and added a test for the attribute-filter function.