A TiddlyWeb plugin that modifies the tiddlyweb_user cookie to add a domain.

The goal here is to allow auth in one subdomain to be useful
in another.

When installed as a system_plugin, any 'tiddlyweb_user' cookie
produced lower down in the stack with a Set-Cookie will have
a Domain attribute added with a value set from

* config['cookie_domain']
* config['server_host']['host']

choosing the first if defined.
