Download 7zr.exe on-demand

[jasongin]

From discussion at https://github.com/jasongin/nvs/issues/5

Currently 7zr.exe is checked in to the NVS repo. It is used to extract .7z archives on Windows, to allow for smaller downloads and faster extraction compared to using ordinary .zip archives.

While it's only 444 KB, putting binaries in git is a bad practice. Instead, NVS could download 7zr.exe before it is needed. Of course that would only be for Windows. (On other platforms NVS uses tar's .tar.xz extraction support if available, otherwise it falls back to .tar.gz archives.)

Currently the only official way to download 7zr.exe is as part of the LZMA SDK. This change for NVS would require 7zr.exe to be posted separately somewhere.