Expand committee to a wider group

Opening from https://github.com/mapbox/tilejson-spec/issues/35#issuecomment-393577842

What are the governance plans for the specification?

For background, the general requirement for an open standard would be a mix of users and implementers, implementers not dominated by one area of implementation at the cost of others, and not dominated by any one organization. This makes sure that an open spec represents everyone's interests, not just one companies.

Hey @pnorman, as stated above, myself (@mapsam) and @GretaCB will be the lead authors and deciding committee of these two versions. We are following the CONTRIBUTING.md, which states all questions and suggestions should be opened up as GitHub issues. Thanks for the input so far!

I'd like to see an open committee to make the standards process openly owned by the community, with a balance of interests. A diverse committee is generally believed to lead to better specifications.

No implementation file or test is identified; read CONTRIBUTING.md and the linked issue discussion first. The requested outcome is an openly owned, balanced committee, but membership and acceptance criteria are not defined in this issue, so the scope must be settled before work can start.