add horizontal divider support to nav list, including documentation for it, per #2106

less/navs.less

@@ -64,6 +64,10 @@
 .nav-list [class^="icon-"] {
   margin-right: 2px;
 }
+// Dividers (basically an hr) within the dropdown
+.nav-list .divider {
+  .nav-divider();
+}