Not support non-systemwide perf result

In the document, it gives an example of command, which works great:
```bash
perf record -a -F 999 -g ./my_program > perf.data
```

But in most case, I don't have sudo privilege, which means I cannot use `-a` to perf systemwide events.
Also the size of systemwide result is really large.
Currently speedscope doesn't support this kind of format. Do you have any plans on this? Thanks!