Badge request: From AppVeyor build artifact

I'd like to display badges based on the contents of an AppVeyor build's artifact files. The build would drop special text files which AppVeyor would report as artifacts. These text files would contain just one line of text:

`--`

The artifact filenames would follow some agreed upon convention such as `SOMENAME.shield`.

The shield could then be displayed via a URL like this:
https://img.shields.io/appveyor/ci-artifact/USERNAME/PROJECT/SOMENAME.svg

See https://github.com/AArnott/pinvoke/issues/24 for a full description of what we want to create.

Start by reviewing the proposed /appveyor/ci-artifact/USERNAME/PROJECT/SOMENAME.svg entry point and the AppVeyor artifact convention described in the issue. Confirm how artifact filenames and their single-line SUBJECT-STATUS-COLOR contents should map to a rendered shield; done means the example URL returns the requested badge.

Written by the indexing model from the issue text.