Modularized binary cache cache

I found this recipe very useful, thanks a lot for creating it! Unfortunately it was a bit too baked into the nixops configuration for me to use directly and I also noticed a few minor issues, e.g. with compatibility with the CloudFront use of HTTPS.

So I made a regular [NixOS module](https://gist.github.com/rycee/f495fc6cc4130f155e8b670609a1e57b) that I use like this:

```nix
{
# …

imports = [ nix-binary-cache-cache.nix ];

services.nixBinaryCacheCache = {
enable = true;
virtualHost = "mycache.example.org";
resolver = "127.0.0.1 ipv6=off";
};

# …
}
```