A quick function to add svgstore icons to WordPress menu items. Just make sure your item class match the symbol definition. Use Modernizr and Css to get a png fallback

WP SVG svgstore Menu Icons + .png fallback

add_filter( 'walker_nav_menu_start_el', 'kk_add_menu_item_icon', 10, 4);

function kk_add_menu_item_icon( $item_output, $item, $depth, $args ) {
  $icon_name = $item->classes[0];
  return preg_replace('/(<a.*?>[^<]*?)</', '$1' . "<svg class=\"icon icon--{$icon_name} icon--svg\"><use xlink:href=\"#symbol-{$icon_name}\" /></svg><span class=\"icon icon--{$icon_name} icon--png\"></span><", $item_output);
}