recharter proposal: 2018-2020

Hey folks.

I've staged a PR with suggested charter updates:
https://github.com/w3c/charter-webperf/pull/37

I believe it should capture all of the suggestions and feedback from
the proposal
doc
<https://docs.google.com/document/d/1Zx2orxWQhjN7fsIfrR2EHVhVzIanppKIyaqewrFJIFM/edit>
and our discussion on last week's WG call. That said, please take a close
scan and let me know if I missed anything, or if there are any additional
changes we should consider.

ty!