add mechanism for managing a consistent set of label across repos, with local overrides

[salewski]

This would work mainly with the GitHub "Labels" API:

• https://docs.github.com/en/rest/reference/issues#labels

A "label" is comprised of a name, a (background) color, and a description.

When displayed in the GitHub webapp UI, the name is printed in either black or white on top of a button-sized area whose background is filled with the label's configured color. Whether the text appears in black or white cannot directly be controlled, but is probably decided based on the L value in the HSL (as in the Farbtastic Color Picker):

    // ...
      $(fb.callback).css({
        backgroundColor: fb.color,
        color: fb.hsl[2] > 0.5 ? '#000' : '#fff'
      });
    // ...

Use cases:

• Setting up a repository: apply a canned set of labels
• Add some new label definition to "all my repos" (with exceptions)
• Diff current set of labels on a repo against a canned config
• Generate labels config from documentation, or vice versa (something simple, though)
  The idea that there is a single source of truth, and it is documented.
  Any file format used should be diff-friendly so that it can easily be seen whether differences exist specifically in the names, colors, descriptions, or some combination.
• Change definition of an existing label (color or description) to match a given spec
• Add canned label Foo to all issues and/or PRs in a repo currently tagged with label Bar