Curious if you'd be interested in some rudimentary hooks

The naming is awful and the code is clumsy at best, will need to change/fix if you're interested in a merge. I'm making the PR to see if you're interested at all.

I did this as part of a (so far generally successful) attempt to add Istanbul code coverage reports to ember-cli.

It would be great if broccoli filters could be leveraged in some way, and it would be great if pigs could fly. For my use case a rudimentary hook did the job. 

Thanks for Broccoli!