Add a JSON release artifact for additions and removals

[ddbeck]

What would you like to see added to BCD?

For each release, I'd like a JSON artifact that contains the release notes' additions and removals. It'd be a structured version of the human-readable release notes.

It's probably too much to ask, but it'd be really helpful to have explicit renames (e.g., key X became key X) as well.

How impactful do you think this enhancement will be?

This would help me do some work automating the task of assigning work in web-features (triggered by BCD upgrades), to cover new features in web-feature as they're added to the web platform. I'd have an immediate application for this.

Do you have anything more you want to share?

I don't need this to be part of the BCD releases themselves (e.g., I'm not expecting something added to __meta). Rather, I'd like to be able to get a separate JSON file from each BCD release on GitHub.