Split template_name into dirname and basename.

I believe this is closer to what we intend the behavior to be. For
example, if you have the following setting:

    JINGO_EXCLUDE_APPS = (
        'admin,
        'myapp/admin',
        'myapp/email',
    )

Then templates in the `myapp/` prefix will be rendered with Jinja2 but
templates in the subdirectories `myapp/admin/` and `myapp/email/` will
be rendered with Django.