Add download attribute for link tag to transfer downloaded filename

[tvolf]

This little update allows sending in link tag encoded file with ability to set this filename in new 'download' attribute for link tag.
We've tried to use this modified version locally for our job and made decision that it will be useful for somebody else.
To describe new ability it needs to add 'download' attribute for link like:

{
  "title": "Link to downloaded file with data in it",
  "type": "string",
  "links": [
    {
      "rel": "anchor name",
      "href": "...here is base64 encoded data string with MIME...",
      "download": "filename for download"
    }
  ]
}

Best regards.
Stan.