Add function use_appveyor() similar to use_travis()

[AppVeyor](http://appveyor.com) is a Windows-based CI system much like Travis CI, and also free for open-source projects. I have prepared [a wrapper around r-travis](/krlmlr/r-appveyor) so that AppVeyor can be used to test R packages (and also later deploy them in binary form). It's work in progress, but I think the format of the `appveyor.yml` configuration file is more or less stable.

Example: [Test output](https://ci.appveyor.com/project/krlmlr/rappdirs/build/1.0.77) for a build of the `rappdirs` package.

Pull request follows.
It looks like this is at least very difficult (if not impossible), since the shield URL contains a unique identifier, as in https://ci.appveyor.com/api/projects/status/xtc629xek00o5rui .