Set up Windows CI with AppVeyor

[pyrrho]

Let it first be said that this PR merits some discussion. Sorry if I should have kicked this off with an issue rather than a PR.

As Travis doesn't yet support Windows, AppVeyor seems to be the go-to alternative. It's free for open-source projects, so this shouldn't add any costs to this project. It will require one of the maintainers of this project to create an account with that service.

This appveyor.yml was based off of https://github.com/starkat99/appveyor-rust, and I used the Linux/OSX matrices as the template for the MSVC matrix, so hopefully those are correct. I'm no AppVeyor expert, so modifications to this script are welcome.

[Cobrand]

This is a welcome change. If you know what do I need to do on my end to add appveyor, that would be really helpful. I'm also not sure it will work right away, the bundled story with windows always has been a bit of a pain point for me, so I don't expect it working on the first try.

[pyrrho]

If memory serves -- I've never actually opened an AppVeyor account -- you should just need to head to https://ci.appveyor.com/signup, select the free-for-opensource plan, and give AppVeyor read-access to your GitHub account. I'm sure there's some more configuration that could be done, but I can't say for sure what's best for this project.

And I agree that windows won't work out of the gate, and definitely not until #796 lands. Even then... I'm not sure if anything but bundled SDL will work, as I'm not familiar with adding system dependencies to those Windows builders. If there's no easy way to add sdl2.dll to the builders' PATH, the non-bundled build configurations could be removed. I'd take some Windows CI coverage over none any day.

[Cobrand]

I sent an Appveyor request to @AngryLawyer , I'll follow up once appveyor is approved for this project.

[sunjay]

Just thought I'd mention this in case it's useful, Travis CI now supports Windows with Rust projects.

https://blog.travis-ci.com/2018-10-11-windows-early-release

[pyrrho]

That is some wonderful new. I guess this PR can be replaced with the addition of - windows to the os: list in travis.yaml, so I'll go ahead and retract this PR.

@Cobrand, would you like me to open a PR for that, or is that a change that you'd rather just push to master?

[Cobrand]

Follow up is at #800. I feel like it's not going to be as simple as adding - windows to .travis.yml since our build config is rather complicated.. but we'll see.