update mapboxgl peer dependency versions, update contributing.md

[chriswhong]

• Updates mapboxgl peer dependency to v1 || v2 so that those using modern versions of mapboxgl will not see a dependency warning.

• Updates example.jsto use streets-v12 style (instead of v9)

• Minor update to contributing.md to show npm commands instead of yarn commands. (Not sure what the right answer here is, but the repo has a package.lock and not a yarn.lock. There is some record in the changelog about switching to yarn to avoid some error but I am not sure that it is relevant anymore)