Auto-tag node core PR's with sub-system label

[phillipj]

Identifying which sub-system(s) PR's relates to by looking at the files changed. This in turn could be used to automatically add sub-system labels, and possibly /cc the correct people/teams listed in docs/onboard-extras.md.

Refs nodejs/node#5710 (comment)

[Qard]

It might be good to detach the auto-cc from the auto-label so labels added manually could also automatically trigger a bot cc'ing the relevant people.

[phillipj]

It might be good to detach the auto-cc from the auto-label so labels added manually could also automatically trigger a bot cc'ing the relevant people.

Good catch, haven't thought of that before. Thanks 👍

[Fishrock123]

closing in favor of #31