download libvips binary during install on windows

[jcupitt]

Many users struggle with libvips install on Windows. pyvips should take a leaf from Sharp's book and download the latest libvips binary during install on that platform.

We'd need to:

• Use the github API to find the latest version -- eg. curl -s https://api.github.com/repos/libvips/libvips/releases/latest > info.json and parse to find the latest win64 asset. We should consider rebuild releases, so we should prefer 8.7.0.1or 8.7.0a to 8.7.0.
• Download the zip, probably the web version. We'd need an install option to select web or all.
• Unzip somewhere inside the pyvips install area.
• On startup, prepend the bin directory in the pyvips libvips to PATH so that we pick up those DLLs.

We could do this on other platforms as well. Perhaps, like sharp, only download if no platform libvips is found. On Linux, we'd need to check libc compatibility, I think.