Add option for custom/extra params in GA create

There is a need for more customization of the GA create call - for myself it's passing an existing clientId to deal with cross domains for example. Mods allow a "params" field to be added to initialization.

    analytics.initialize({
      'Google Analytics': {
        trackingId: 'UA-XXXXXX-0',
        universalClient: true,
        params: {
          clientId: '111',
          storage: 'none'
        }
      }
    });