Add speedIndex metric

[andrewrota]

The RUM SpeedIndex metric calculates speedIndex (an indexed value representing the average time at which visible parts of the page are displayed) from resource timings. Is this something that browser-perf would be interested in adding as a builtin metric?

I have an initial implementation here.

[axemclion]

👍 Awesome - adding speed index would be great. Could you send a PR and we could colloborate over that ?