Add `pTraceWith`

[georgefst]

If haskell/core-libraries-committee#36 gets accepted (and arguably, even if it doesn't), we should add analogous functions to Debug.Pretty.Simple.

I end up defining traceShowWith/pTraceShowWith myself all the time. It's a very useful function.

[cdepillabout]

This looks like it was closed in #104.

Thanks!