Add AppVeyor configuration

[wilzbach]

Just noticed that there wasn't a .appveyor config yet - as usual you need to add the project on AppVeyor. If you plan to move it over to the vibe-d organization (?), it might make sense to do this afterwards.

(this is untested, but it's a variant of a script that I wrote a year ago and gets copy/pasted around)

[wilzbach]

Ping @s-ludwig - how about enabling AppVeyor for diet-ng?