Use field.id_for_label instead of field.auto_id

This fixes a bug when specifying a custom id for a field and rendering
it with the `|bootstrap` filter.

When using `field.auto_id` in the label's `for` attribute, this is
always set to the default value instead of the custom `id`.

bootstrapform/templates/bootstrapform/field.html

@@ -46,7 +46,8 @@
         </div>
     {% else %}
         {% if field.auto_id %}
-            <label class="control-label {{ classes.label }} {% if field.field.required %}{{ form.required_css_class }}{% endif %}" for="{{ field.auto_id }}">{{ field.label }}</label>
+            <label class="control-label {{ classes.label }} {% if field.field.required %}{{ form.required_css_class }}{% endif %}"
+                   for="{{ field.id_for_label }}">{{ field.label }}</label>
         {% endif %}
 
         <div class="{{ classes.value }} {% if field|is_multiple_checkbox %}multiple-checkbox{% endif %}">