feat: cookie_domain config to support subdomains

[andrewjtait]

By default, Authie sets cookies without an explicit domain, which scopes
them to the exact host that issued them. If you need a single session to
be valid across multiple subdomains (for example app.example.com and
admin.example.com) you can configure a cookie domain. Both the
user_session cookie and the browser_id cookie will then be set with
this domain.

Authie.config.cookie_domain = '.example.com'