Support pprof's profile.proto format

[Timmmm]

It would be cool if Samply supported the format used by pprof because that has by far the best visualisation of profiles I've ever used (as long as you don't care about time-varying performance).

[Timmmm]

Here's the format, it's very simple.

In fairness the Firefox Profiler is pretty good too, but pprof definitely has some better features (e.g. when you hover a function in the flame graph it highlights all calls to that function, and if you click it it groups those calls together).