Generate parseable JSON file with a history of channel updates

[domenkozar]

For the sake of tracking NixOS release process I think we should provide a way to introspect the old releases. This used to be visible before S3 as we just had folders.

Given the current pipeline using these scripts we could simply append a JSON file http://nixos.org/channels/history.json with the following format:

{
  version: 1
  channels: [
     {
       name = "nixos-16.09-small",
       date = "2016-11-16 06:08:34";
       commit = "https://github.com/NixOS/nixpkgs-channels/commits/6270733155c381090fc5c7de6bddc26fbf35f47f",
       url = "https://d3g5gsiof5omrk.cloudfront.net/nixos/16.09/nixos-16.09.1032.6270733",
       hydraUrl = "https://hydra.nixos.org/eval/1304255",
     },
     ...
   ]
}

cc @edolstra @rbvermaa

Open questions:

• how to deal with the file once it gets bigger

[domenkozar]

@edolstra do you have some immediate feedback/plans for this?