Test Windows builds with AppVeyor

[wilzbach]

A first experiment with enabling a CI for windows. Ideally should be enabled for all repos.
AppVeyor supports cronjobs as well.
This could be made faster if we run the x86 and x86_64 tests in one run, but then the main overview table would be less informative (I am fine with both ways).

[wilzbach]

Wohoo 🎉
My tests on my fork (to avoid spamming with rebases) have worked:

[wilzbach]

So do we go with testing on Windows?
If wanted we can always execute the build.bat (if existent for a repo) as well.
@rikkimax this might be a good starting point to automatically generate the binary executables for Windows.
After the build has succeeded you could let AppVeyor execute a simple D script that builds all binaries, e.g.

dub fetch --version="~master" dfix && dub build dfix
dub fetch --version="~master" dfmt && dub build dfmt
dub fetch --version="~master" dscanner && dub build dscanner
dub fetch --version="~master" dcd && dub build --config=client dcd && dub build --config=server dcd

and then upload them to s3. For more user convenience, you could even bundle them in an installer

[wilzbach]

Ping @ZombineDev @bbasile @Hackerpilot

[ghost]

I've already given my opinion in the community discussions. The plus would be when the build.bat can be tested. since they tend to be less maintained.