Add ifnotswitch template tag

[mrfuxi]

Add ifnotswitch to handle situation like that:

{% ifswitch replace_old_functionality_123 %}
    {# DO NOTHING. Apparently doens't exist an ifnotswitch template tag #}
{% else %}
    <a href="/some/old/stuff">Something old</a>
{% endifswitch %}

...

{% ifswitch replace_old_functionality_123 %}
    <a href="/some/NEW/stuff">Brand new functionality located in different place</a>
{% endifswitch %}

Comment (example) taken from real code

[mrfuxi]

Is anyone interested in this PR? @mattrobenolt