add settings for cookie domain.

In some use cases there is a need to set the domain of the cookie to ".yourdomain.com". (to support all subdomains)
Only a small change is needed to support the __cookieDomain__ option.

You can use that config like other options in the i18next.init function.

    i18n.init({
        cookieName: "lang", 
        cookieDomain: ".yourdomain.com",
        ns: { namespaces: ['ns.common', 'ns.special'], defaultNs: 'ns.special'},
        resSetPath: 'locales/__lng__/new.__ns__.json',
        saveMissing: true,
        debug: true,
        sendMissingTo: 'fallback'
    });

There is no need of a value fallback in the code because the domain option is ignored when it's null or undefined.