adding an alternative for cross-domain issues

I found the cross-domain fix using a proxy tedious to integrate into my project (due to how we setup all our code for our CDN etc) so I came up with a different solution;  
Just fetch the same stylesheet again, without using the CDN.  

I'm okay with a small percentage of my users not using the CDN and it makes my life A LOT easier :-).

Instead of adding a lot of 'magic' I just added a new attribute on my `<link>` called `respond-href` with the URL to load instead, keeping it explicit, though adding an 'invalid' attribute, but I think Respond is mostly for people who're building a HTML5 website anyway, so you won't be passing the old validators anyway?

```html
<link href="cdn.domain.com/statics/stylesheet.css" respond-href="/statics/stylesheet.css" />
```