Add first/last classes on li in menu

This patch includes a settings form for Superfish module. This form include option - add first/last classes for li elements. Form settings are useful for modifying the module in the future.

Exists another way for this issue - change only superfish-menu-items.html.twig file
We can use "special variables" loop.first and loop.last
Example:
{% for elem in arrMenu %}
{% if loop.first %}
// some html element with "first" class
{% elseif loop.last %}
// some html element with "last" class
{% else %}
// some html element without classes
{% endif %}
// some html structure
{% endfor %}

If you need this I can add this way in the new patch