Add support for setting the cookie configuration for analytics.js

[pgrippi]

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:

ENV.googleAnalytics = {
  webPropertyId: 'UA-XXXX-Y'
};

if (environment === 'development') {
  ENV.googleAnalytics.cookieDomain = 'none';
}