Added "normalize_whitespace" filter

Use case:

```jinja
{# <title> generation #}
{% block title %}
    {%- filter normalize_whitespace %}
        {% if foo %}
            {{ foo.title }}
			{% if bar %}
			    / {{ bar.title }}
			{% endif %}
			/
        {% endif %}
         / {{ parent() }}
    {% endfilter -%}
{% endblock %}
```

TODO:
- [ ] add documentation if this PR can be accepted
- [ ] consider adding "spaceless" filter (IMHO actually it is filter, not tag)