Add support to type, media and crossorigin attributes

[andrevandal]

Hi.

According to Netlify post, link path should have rel and as and Fonts should crossorigin tag with two acceptable values (anonymous or use-credentials). (https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link#attr-crossorigin)

And a type and media attributes also should be acceptable. (Ref: https://developer.mozilla.org/en-US/docs/Web/HTML/Preloading_content)

Exemple:

Link: </js/example-script.js>; rel=preload; as=script
Link: </font/example-font.woff2>; rel=preload; as=font; crossorigin=anonymous
Link: </images/bg-image-narrow.png>; rel=preload; as=image; media='(max-width: 600px)'

Looking in transformer.js:

module.exports.netlify = (headers, headersFile) => {
  const netlifyHeader = headers.map(
    ({source, headers}) => `${source}\n\tLink: ${headers}`
  );
  fs.appendFileSync(headersFile, '\n' + netlifyHeader.join('\n'));
}; 

I think the only solution is test headers extentions to add as (and crossorigin to fonts) based on file's type.
What do you think?