add customizations to datespan filter

Not sure what you mean by "just change it to templates". I was thinking of something like this:

```html
    {% if show_startdate %}
    <label for="{{ css_id }}_startdate" class="sub-label">{{ default_from_text }}</label>
    <input type="text"
        id="{{ css_id }}_startdate"
        name="{% ifnotequal slug 'datespan' %}{{ slug }}_{% endifnotequal %}startdate"
        class="date-picker input-small"
        value="{{ datespan.startdate|date:"Y-m-d" }}">
    {% endif %}
```