Add Meta Information about Generators into Source Map

[mitsuhiko]

Today if you have a broken source map and a customer files an issue, it takes a few back and forth until it's clear how this source map has been created. It would be nice to have a place in the source map where generators can leave information that they manipulated or created it, and with which flags and version.

(Originally brought up by @kamilogorek)

[bmeurer]

This is a great idea, I'm very much in favor of this.

[jkup]

This sounds great! One additional thing was that It sounds like folks have particular difficulties when multiple generators are run to generate the final source map. So ideally, however we store this would have room for an ordered list we could trace back.