Github nightly build page

[Nick]

Thanks to some excellent work by Gauss (I believe), fosskers on Github and the continually awesome backwardsEric, we once again have an automated process for building the latest development code on both Windows and macOS.

To get the latest builds, simply go to the release page on Github, open the "Assets" section at the bottom of the top entry, and there will be download links for Windows and macOS builds and source. Amazing!