Run CI on additional platforms incl. Windows

[marcysutton]

To catch issues like #516, we should run our tests on additional platforms, starting with Windows. We could use Sauce Labs through CircleCI for this. It would allow us to catch installation and test problems with minimal effort. https://circleci.com/docs/1.0/browser-testing-with-sauce-labs/

[nschonni]

I'd suggest https://www.appveyor.com/ so you get a full install experience since you're a developer tool.

SauceLabs is 💯 better for testing the cross platform browser stuff though.

[marcysutton]

Closed with #545.

[WilcoFiers]

Reopening this - we're not actually using AppVeyor right now, since it's been causing some build issues. We need to resolve those.