Combine simple_tag and assignment_tag

[Jimmy Merrild Krag]

It would be awesome if simple_tag could used as an assignment_tag as well, such that there were two forms of uses of the same tag:

{% mytag argument1 argument2 %}
and
{% mytag argument1 argument2 as variable %}

Where the first function as simple_tag does now and the second as assignment_tag, but both are defined by the same function.

[Tim Graham]

This is done as of Django 1.9 (#18651), and assignment_tag is deprecated per the ​1.9 release notes:

Django 1.4 added the assignment_tag helper to ease the creation of template tags that store results in a template variable. The simple_tag() helper has gained this same ability, making the assignment_tag obsolete. Tags that use assignment_tag should be updated to use simple_tag.