A badge for build stability

[binkley]

This is part of the epic #586.

I would like to show a badge on the project for build stability: what has been the recent pass/fail rate, and bucket those into good descriptive words.

This will help teams visually spot when a project is going through rough times (could be the code, could be the environment).
Ideal would be to have knobs to tweak to configure. Example:

• Green if last 5 builds were green
• Amber if last 5 builds were a mix of red & green
• Red if last 5 builds were red

An example with the right approach is https://github.com/dwyl/repo-badges but it is unclear how to use in this project.
See dwyl/repo-badges#53.