Add a `sassLibs` option to the options has for `new EmberApp()`.

This allows vendored sass libs to be passed into the ember-cli precompilation process.

I hope this PR can be a discussion starter for how this use case might be handled in ember-cli.  This is the simplest thing that I could get to work, and it currently has no tests.  If this approach seems OK I'm more than happy to write some tests around it, and if a different approach would be better I'd be happy to work on that.

Usage:
When creating the `EmberApp` in `Brocfile.js` you can pass a `sassLibs` option, which should be an array of paths that contain sass/scss files.

```javascript
var app = new EmberApp({
  //...
  sassLibs: ['vendor/bootstrap-sass-official/vendor/assets/stylesheets']
});
```

This would make the bootstrap sass files available to be included in `app.scss`.

```css
@import 'bootstrap';
```