Allow overriding of theme_menu_link()

I need to override the theme_menu_link() function from my theme, however I can't as your module is replacing it with a custom, non-themeable function...

Since the functionality provided by your custom function can be achieved in menu_css_names_preprocess_menu_link() (that you're already using), I propose removing the custom replacement for theme_menu_link() and moving the code into menu_css_names_preprocess_menu_link().