Heat Map support

[Sergey Chernyshev]

Merge support for Heat Map calculations based on my talk here:
https://speakerdeck.com/sergeychernyshev/using-heat-maps-to-improve-performance-metrics

and some prototype code in this project:
https://github.com/sergeychernyshev/speed-selector

We can have Heat Map defined as a JSON structure that can be seen as "selector-boundaries" custom metric on this test:
http://www.webpagetest.org/result/160411_T1_16ZW/1/details/

(gathered using this custom metric script)

Support for custom thresholds within heat map are in #15

[Sergey Chernyshev]

NYWebPerf team will take a stab at this during one of the Meet4SPEEDs or just remotely.