support hot/cold flamegraphs

[Lucas Kent]

I have not actually used these before but they are supposedly very useful.
Refer to https://www.brendangregg.com/FlameGraphs/hotcoldflamegraphs.html

[Jon Gjengset]

That sounds like a great idea! I think this is mostly a matter of how the input is collected, and then just figuring out which frames to use red-ish/blue-ish for depending on something in the output of perf script. Would be really neat indeed!