Added more tags to common_helpers.

Of note are:
    * select_article and select_section, both make a selection
        based on the object's name, using the get method. So,
        using these tags require an existing item.
    * paginator_create, creates a Paginator instance in context
        called "paginator_instance"
    * paginator_page, which implements paginator_instance.page(x)

Also tweaked assign_to_context so that values can be converted to
the desired type. But that is entirely optional.