fix(MdList): add active style

Author: marcosmoura

src/components/MdList/MdListItem/MdListItem.vue

@@ -26,7 +26,7 @@
 
       return createElement('li', {
         on: listeners,
-        staticClass: 'md-list-item ' + data.staticClass
+        staticClass: 'md-list-item'
       }, [
         createElement(listComponent, {
           staticClass: 'md-list-item-container md-button-clean',

src/components/MdList/theme.scss

@@ -4,6 +4,10 @@
 
     .md-list-item-container {
       @include md-theme-property(color, text-primary, background);
+
+      &:active {
+        @include md-theme-property(background-color, divider, background);
+      }
     }
   }
 }