Add support for setting the cookie configuration for analytics.js

Fixes #1 

Adds support for setting the cookie domain, name, and expires properties on the `ga` object.

For example, one may turn off cookie tracking for development in `environment.js`:

```js
ENV.googleAnalytics = {
  webPropertyId: 'UA-XXXX-Y'
};

if (environment === 'development') {
  ENV.googleAnalytics.cookieDomain = 'none';
}
```