Add callback function "onBeforeAdd"

- Remove onFreeTaggingAdd
- Add new callback function "onBeforeAdd"

Example for onBeforeAdd

if new a token add prefix on hidden value
 aaa,new[token-value],bbb,ccc


    onBeforeAdd: function(item, isNewToken)
    {
       if ( isNewToken === true )
         item[this.tokenValue] = "new["+ item[this.tokenValue] +"]";

      return item;
    }