Merge options.attr into listAttributes to allow for control through knp_menu_render

If options.attr is provided, it will be merged into list.childrenAttributes, allowing you to override attributes on the wrapping element in a call to knp_menu_render.

Example:
{% set myMenu = knp_menu_get('myMenu', [], { foo: 'bar' }) %}
{{ knp_menu_render(myMenu, {
    attr: {
        title: 'My menu!!'
    }
}) }}
knp_menu_render(main