`{% with %}` template tag

with tests and docs

ready-for-checkin as per discussion on django-dev.

(In [4830]) Fixed #3826: added a {% with %}. Thanks, SmileyChris?.

To use the with tag for more than one variable I need to nest the tags as follow:

{% with var1 as name1 %}
{% with var2 as name2 %}
{% with var3 as name 3 %}
...
{% endwith %}
{% endwith %}
{% endwith %}

This is ugly, cost much typing and can be such confusing, I've done a patch to use one with tag to many variables I want to define, see:

{% with var1 as name1 and var2 as name2 and var3 as name3 %}
...
{% endwith %}

Michelts, I'll leave this ticket open, but in the future if you are suggesting enhancement to something that has already been fixed then please open a new ticket.

The more compact form is not greatly more readable. Let's leave it as nested tags.