Enable hiding of categories from foxyshop_simple_category_children()

Changed the foxyshop_simple_category_children() and foxyshop_simply_category_writer() so empty categories can be hidden.

Not sure if this is the best way, just adding on another (optional) argument to the function.  Seems like it might be good to alter it so that the third optional argument is an array that are the args that can be passed in to the get_terms() function.  That way the user can tweak to their heart's content what categories are returned :)

Let me know if you prefer that way, or if you feel this is sufficient.