[patch] Add new template tag to test 'foo in bar'

[tomevans222]

There is currently no way for a template to test whether a value is in a list or not.

This is quite a useful tag to have, when drawing tables for example:

   ctxt = { 'my_colours': ['orange', 'yellow'], 
            'all_colours': ['red', 'orange', 'yellow', 'green', blue', 'indigo', 'violet'] }

 <tr>{% for clr in all_colours %}<th>{{ clr }}</th>{% endfor %}</tr>
 <tr>
   {% for clr in all_colours %}
   <td>{% IfInList clr in my_colours %}I like it{% else %}I hate it{% endif %}</td>
   {% endfor %}
 </tr>

Attached file adds functionality, but (as I need this now) it follows the style I use for my supplementary tag libraries (camel case node name, 2 space indents, standalone) so would need to be polished slightly to go in django/template/defaulttags.py

[tomevans222]

Implementation of IfInList tag

[Karen Tracey]

ifin and ifnotin already proposed by #8087.