Template tag for loading URLs from specific namespaces

In addition to the template tag ``load_djng_urls``, this PR includes a template tag for loading specific namespaces. When used without arguments, it loads all URLs from the namespace of the current request. Specific namespaces can be included by adding a comma-separated enumeration of strings, e.g.
````
{% load_djng_namespace_urls 'namespace1, namespace2' %}
````
loads the URLs prefixed with ``namespace1:`` and ``namespace2:``.
Possibly the tag could be renamed to something shorter.