All URLs provided by this factory have the subdomain set to a
specific domain. For example if you want to use the subdomain for
the current language this can be a good setup::
url_map = Map([
Rule('/', endpoint='#select_language'),
Subdomain('<string(length=2):lang_code>', [
Rule('/', endpoint='index'),
Rule('/about', endpoint='about'),
Rule('/help', endpoint='help')
])
])
All the rules except for the ``'#select_language'`` endpoint will now
listen on a two letter long subdomain that holds the language code
for the current request.