def append(*values, to: nil)
      keys = Array(to)
      set(*keys, value: Array(fetch(*keys)) + values)
    end
def append(*values, to: nil)
      keys = Array(to)
      set(*keys, value: Array(fetch(*keys)) + values)
    end
Append values to an already existing nested key

 @param [Array[String|Symbol]] values
   the values to append

 @api public