forloop should have an accessor for the total count of the loop

[Jonathan Ströbele]

The forloop has no way for accessing the total loop iteration count, though the ​length of the values being iterated is already known on the logic for building up the forloop variable.

I propose to expose this count on the forloop variable, i.e. like: forloop.total. I think it can be helpful to have this information available inside a loop if needed (this also the case for example in the ​Laravel/Blade loop variable):

{% for val in values %}
  {{ forloop.total }}
{% endfor %}

I prepared a patch here and can send it of as a PR if it's okay: ​https://github.com/stroebjo/django/commit/0a6233bcd9f2356a2b7b646ad4448ee313dc4d43