Select Lighthouse version / master branch preview

[ThatGuySam]

It would super awesome to enable generating reports using the current Lighthouse version as well as previewing upcoming scores from the Lighthouse master branch so you can get an idea of what your scores will be in the next release and take action accordingly.

[jonchenn]

Hi @ThatGuySam, thanks for submitting this FR. This is a great idea, and we'll check if we can incorporate this into the standalone Lighthouse gatherer implementation.