Add debugging output

[Timmmm]

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).