AppVeyor-D: Project to track recommended appveyor.yml (Win CI)

[Nick Sabalausky (Abscissa)]

I wanted to use AppVeyor to add Windows CI to some of my D projects, but 
when I tried the appveyor.yml config on the DWiki[1], I found it needed 
some work.

So after quite a bit of tinkering (and throwing semi-random bits at the 
wall to see what would work), I finally got it running properly in my 
project: including support for older/newer versions of DMD/LDC in both 
32/64-bit.

Then, I merged in some improvements DUB had made, plus laid some 
groundwork for download mirrors (just needs URLs now), and pushed it up 
as a project on GitHub for all to steal:

https://github.com/Abscissa/AppVeyor-D

One nice thing about this (compared to the DWiki page) is that its 
actually being tested by AppVeyor itself. Plus, it now has full GitHub 
Issues/PR support for managing enhancement/submissions/discussion, all 
in one central place.

[1]: https://wiki.dlang.org/Continuous_Integration