adds borderless variant

docs/menu.html

@@ -182,7 +182,7 @@
         <a><i class="icon heart"></i></a>
       </div>
       <div class="right menu">
-        <div class="fitted item">
+        <div class="fitted borderless item">
             <input class="ui input" type="text" placeholder="Search...">
         </div>
         <div class="dropdown item">

src/ui/flat/menu.css

@@ -553,6 +553,20 @@
   padding-bottom: 0em;
 }
 
+/*--------------
+   Borderless 
+---------------*/
+
+.ui.borderless.menu .item:before,
+.ui.borderless.menu .item > a:before,
+.ui.borderless.menu .item .menu .item:before, 
+.ui.borderless.menu .item .menu .item > a:before,
+.ui.menu .borderless.item:before,
+.ui.menu .borderless.item > a:before {
+  background-image: none;
+}
+
+
 /*--------------
      Tight 
 ---------------*/