Proof-of-concept for adding callbacks to handle removal of elements/attributes

It's me again :)

I was just playing around with adding hooks to inspect when an element or attribute is removed. I'll be using this functionality for an upcoming feature, and I wanted to see if you think this might be useful. 

An alternative would be to take blocks, like this:
    ```Sanitize.clean!(html, config) { |node| ... }```
But that required more extensive changes, and only want to pursue that if you think it's a good idea. Thoughts?

Thanks for you time,
Ben