[Feature request] Include Package Diff link in npm PRs

[tlhunter]

I'm a maintainer of Package Diff. This tool provides a diff between two versions of npm modules and has a permalink to display the diff.

We were recently made aware of this thread in which an automated tool included a link to Package Diff. This notified the author / Automattic that a package upgrade wasn't a wise decision.

I believe including a Package Diff link in npm-related greenkeeper PRs would benefit the OSS community by increasing visibility of npm module changes.