Reintroduce support for .scss files

- Reverts the commit that removed Sass support.
- Also fixes broken tests by updating the fixtures to match new asset url pattern
I'm not familiar with `node-sass`, is `renderSync` a blocking call? If so, could we do something like:

```js
opts.success = function scssSuccess(scss){ cb(null, scss) }
opts.error = cb
sass.render(opts)
```

Do we need to do this to get render errors anyway?