Labeling PRs from external contributors

[tylerbutler]

Would you be interested in adding the capability to label PRs based on the memberships of the user who opened it? In projects that have a core contributor team that merge their own PRs once approved, it can be useful to flag PRs from users who don't have merge permissions. Such PRs need to be merged by someone in the core team.

I am willing to open a PR to add this functionality if you're open to adding it.

[kaxil]

Definitely open to adding it if you can submit a PR