Instrument filters in the pipeline

This makes `HTML::Pipeline` compatible with [ActiveSupport::Notifications](http://api.rubyonrails.org/classes/ActiveSupport/Notifications.html) instrumentation.

Shuffles things around a bit but adds instrumenting to every filter call.

@jch I know we talked about putting the instrumentation on the individual filters but I decided to keep it here since I wanted to make it easy to make the instrumentation service object available. The pipeline felt like the right place to put that, and the individual filter wasn't aware of the pipeline object as far as I could see.