Add editor-image-subtitle class to image subtitle html tag

• Add editor-image-subtitle class to image subtitle markup. When used on the web, you can now style the tag as for eg:

   .editor-image-subtitle {
   color: #828282;
   font-size: 10px;
   text-align: center;
   display: block;
   margin-top: 7px;
   margin-bottom: 25px;
   }