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!
The beforeOutputPush afterEval callback never gets called. 

Perhaps add the wrapInEval val to the ops passed to the after callback. That way the callback can decide what, if anything, it wants to do in both scenarios. Would probably have a different name in that case.