Add speedIndex probe and metric

[andrewrota]

Adds probe and metric for RUM SpeedIndex, an indexed value representing the average time at which visible parts of the page are displayed) from resource timings.

Related issue: #69

[andrewrota]

I'm going to refactor this to pull in the rum-speedindex package as a dependency

[axemclion]

@andrewrota Sorry for not having responded sooner - I think making it a separate package would be better. That way, browser-perf and other tools just supply the information, and hte actual number can be calculated in a more common way.

[andrewrota]

@axemclion Do you mean pulling in rum-speedIndex as a dependency (like this master...andrewrota:57bd61b985372d895763db85d5b3a5f36aa91349) or packaging the entire speed index metric separately as a custom metric + probe?

[axemclion]

@andrewrota Sorry for a late response, had been busy with some conferences.

Pulling the metric + probe may not be as useful, and others cannot include that module anyway - it is too browser-perf specific.

I meant more like the rum-speedindex part. The RUM-SpeedIndex was what I was talking about - that is great.

[axemclion]

@andrewrota Hey, did you need any other info on the PR ?

[andrewrota]

@axemclion Sorry, wasn't sure what your last suggestion was. This patch is still what I'm thinking: pull in rum-speedindex as a module from npm and inject it into the page with a probe, then use a metric to record the data.