Add Windows to CI matrix

[kazuho]

By adding Windows to the flavor, we can prevent the build from becoming corrupted repeatedly on Windows.

See also: #349

[huitema]

I am not sure that Travis has an option for running on Windows. In picoquic, I do that by running tests for Windows using appveyor. Would appveyor work for you, or do you prefer to use github actions?

[kazuho]

I do not have a strong opinion, though I think we can have more eyes to maintain the CI integration if we use GitHub actions, as we use it for h2o already.

[huitema]

I have started writing a github action that compiles the Visual Studio solution -- see PR #352. Currently this fails, because the build requires access to OpenSSL libraries and headers -- and the exe requires access to openssl dll.

I suppose I could write a script that downloads the whole tarball, as well as the required version of perl, compiles it, and puts it in a location that the windows build can use. But that's quite a bit of scripting... I wish someone has done that already, but i can't find it.