Support for boolean tags

It would be nice to have an easier way to create boolean tags.

Template:

```
{% ifequals thing1 thing2 %}
All Things are created equal
{% else %}
Things are not equal
{% endifequals %}
```

Tag:

```
@boolean_tag(register, [Variable(), Variable()])
def ifequals(context, thing1, thing2):
return thing1 == thing2
```

I took a stab at it in my fork.
seanbrant/django-templatetag-sugar@ea353e5e50152fe5a353e5aa7fff1fdbb76cf2e0