Add Windows-based CI

[niieani]

Since some people want develop with Aurelia under Windows it makes sense to also have a Windows-based CI. AppVeyor seems free for open-source projects and looks rather simple to configure, like CircleCI which we already use.

Ref: https://www.appveyor.com/docs/lang/nodejs-iojs

I'll deal with this together with the Windows fix for webpack-plugin.

[Vheissu]

Does the Aurelia CLI negate the need for this now @niieani or this is something separate?

[niieani]

Separate. It'll ensure skeletons build under Windows and Linux/macOS.

[EisenbergEffect]

@niieani Did you end up adding this already?

[niieani]

No. I've researched the topic though. CircleCI can only do tests on Linux and Mac, but not on Windows. But there's Appveyor that is free for Open Source and does tests on Windows. We should add it so we'll know if a pull request breaks Windows support (as it happened before with the webpack skeletons, since most of us runs OS X).