Configurable cookieDomain option for ember-simple-auth-cookie-store

Hi! This need to share session with subdomains. if not specified, cookie domain defaults to the host portion of the current document location (e.g. example.com). If you need to share session data with your subdomains (store1.example.com, store2.example.com) you need to set session domain as .example.com.

```js
window.ENV = window.ENV || {};
window.ENV['simple-auth-cookie-store'] = {
  cookieDomain: '.example.com'
}
```
I'd say this should not be set at all when `this.cookieDomain` is empty (see how `expires` is set).