Allowed additional where conditions for update

The 3rd, optional parameter to `updatePivot` basically exposes all `builder` methods, but should primarily be used for adding custom `where` conditions to the update query. Enables users to do the following:

Model.forge({id: someId})
.someRelation()
.updatePivot({additional_column:'new data'}, null,{whereIn: ['info_id', anotherId]})
.then(function (col) {
  console.log('updated');
});