Add contributors check

[atesgoral]

Add a gulp task to parse contributors YAML and to emit it as JSON. Check author names in filenames against the contributors list and fail the build if a PR doesn't add a newly-introduced author into contributors.yml.

Do some archeology and fix the newly failing contributors check 🎉

[clarfonthey]

If you're just going to be reparsing the file into JSON, is there a reason why the original has to be YAML in the first place? I get that it's easier to edit, but it's not that much harder to edit JSON.

[atesgoral]

@clarfonthey You're absolutely right! I was thinking that YAML is potentially easier to edit, especially for newcomers, but it's not even true. YAML can be really finicky. I'll tweak this PR to switch to JSON and remove the messy complexity.

[atesgoral]

@clarfonthey merging!