TeeResponseWriter for various post-processing tasks.
Also add TeeHeaderResponseWriter.

Oftentimes, post-processors won't care about the body so we can be more efficient with our memory.
Use TeeHeaderResponseWriter in CounterByStatus(XX).
s/Status/StatusCode/ to be like the standard library.
Refactor PostProcessor to use TeeResponseWriter.

This is a little less flexible than the PostProcessor+ResponseWriterWrapper style but I think in such cases custom handler is essentially necessary anyway and such handlers will have Tee(Header)ResponseWriter available to make their lives easy.
Fix the example; broken since 5f436e4.