Allow array of paths to the preprocessCss phase

```js
var app = new EmberApp({
  preprocessCss: {
    options: { 
      paths: [ 'main', 'theme/a' ]
    }
  }
});
module.exports = app.toTree();

// app/styles/main.scss    -> dist/assets/main.css
// app/styles/theme/a.scss -> dist/assets/theme/a.css
```

*Ensures backwards compatibility* so that when no paths are configured; `app/styles/app.scss` will still be written to `assets/<app name>.css`

Closes #1656.
I was trying to emulate a pattern I saw in [other options hashes](https://github.com/tstirrat/ember-cli/blob/multiple-css-files/lib/broccoli/ember-app.js#L86) but in hindsight those are build stages that be enabled or disabled. I have no strong opinion one way or another. Happy to move them up into `preprocessCss`.