Add debugging output

I currently have an issue on OSX where nearly all of my profiles have "" as most of the time. This is obviously something to do with `llvm-symbolizer` not working, maybe some `dSYM` bundles are missing or something.

Unfortunately this is almost impossible to debug because `pprof` doesn't include any verbose debugging options so I can (for example) see what it is sending to `llvm-symbolizer`. I can't see which files it is opening, where it is looking, etc. etc.

I would suggest adding something like the standard `-debug=none/error/warn/debug/info` type thing. For now I have resorted to adding `fmt.Printf()`'s into the code (fortunately Go is nice and easy to recompile).