added {% spaceless %} tag support

I find the {% spaceless %} tag useful in Django and thought I'd contribute it back as a default tag. 

### Included:
 * {% spaceless %} tag added as default tag
 * tests for leading, trailing, and "between-tag" whitespace trimming.

### Use:

```django
{% spaceless %}
    <li>1</li>
  <li>2</li>
        <li>    </li>
<li> 
4
</li>

{% endspaceless }%
```

```html
<!--// BECOMES //-->
<li>1</li><li>2</li><li></li><li>
4
</li>
<!--// 
    notice it only removes empty whitespace 
    so "4" retains it's original whitespace 
//-->
```

### Notes

I borrowed the "between-tag" whitespace stripping RegEx `>\s+<` directly from the [Django {% spaceless %} function](https://github.com/django/django/blob/master/django/utils/html.py#L135-L138) and added leading and trailing whitespace trimming which doesn't appear to be a function of the Django function (but perhaps should be?)