allow for deleting items from sets

allows for deleting elements from sets with syntax like:




````
// Given a dynamo item:
{
 someattr: ['foo', 'bar', 'baz']
}

// Delete like:
{
            delete: {someattr: ['foo', 'bar']}
}


// Resulting item in dynamo:
{
 someattr: ['baz']
}

````