set htmlClass on tab elements if defined

src/tabs.html

@@ -1,13 +1,14 @@
-<div ng-init="selected = { tab: 0 }" class="schema-form-tabs {{form.htmlClass}}">
+<div ng-init="selected = { tab: 0 }" class="schema-form-tabs {{::form.htmlClass}}">
   <ul class="nav nav-tabs">
     <li ng-repeat="tab in form.tabs"
         ng-disabled="form.readonly"
         ng-click="$event.preventDefault() || (selected.tab = $index)"
-        ng-class="{active: selected.tab === $index}">
+        ng-class="{active: selected.tab === $index}"
+        class="{{::tab.htmlClass}}">
         <a href="#">{{ tab.title }}</a>
     </li>
   </ul>
 
-  <div class="tab-content {{form.fieldHtmlClass}}">
+  <div class="tab-content {{::form.fieldHtmlClass}}">
   </div>
 </div>