Add ** operator

[gilbertbw]

This stops e.g. the following code from showing a readability warning:

defmodule CredoTest do
  @moduledoc """
  An example of the snakecase test failing
  """
  def a ** b do
    nil
  end
end

[rrrene]

@gilbertbw Thx! This is part of Credo 1.7.9 🎉