Bring back pluggable reporters

[mtscout6]

From #63 (comment)

css-band-aid did not account for customizable reporters. It's a great idea and makes testing easier, though it will need some more thought since there's also an additional command to count selectors now. Both the chunker and counter could use custom reports.

[aabenoja]

@paulyoung How important is it to get this into v4?

[paulyoung]

I think it'd be a shame for it not to make it in since it was previously a part of the 4.0 branch, and the additional test coverage that came when I did my original implementation added some confidence.

Provided that bringing it back won't introduce any breaking changes I could be persuaded to shelve it. I'd love to see it in something like 4.1 though.

[mtscout6]

I think 4.1 sounds reasonable for this. We could easily implement this in a backwards compatible way. I'm going to move this to the 4.1 milestone.