Non-blocking CSS loading

By default CSS files loaded through `<link>` tags are downloaded synchronous, while an asynchronous approach would be preferred. 

By setting the media attribute of the link tag to a non-matching item the browser will treat it as non-blocking, making the request async. Once the CSS has been loaded the media attribute is reset to `all` so it will match and the CSS will get applied.

This is based on information found at: https://gist.github.com/scottjehl/87176715419617ae6994