Add support for flamescope

https://github.com/Netflix/flamescope has pretty nice trick up it sleeve, which I'd like see integrated into pprof. I can see this feature combining 3 parts:

* time/offset based filtering for flamegraph (or better yet for all visualization). This could be achieved with two optional GET parameters for web visualization, e.g. `start_offset` and `end_offset` or camelCase or whatever. I can work on PR for that
* Preparing data serverside for bucketing into X-ms slot buckets. I can also work on PR for that probably.
* Frontend GUI integration.....this thing is harder for me to work on since I'm primarily in the backend/DevOps world.

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I already have experience using flamescope and it paid its learning curve within first hour when I found bug easily which has been hidden in typical flamegraph view (( systems bug, thus perf input data, not pprof ))