Make it possible to show testsetup blocks

[tsznuk]

Testsetup blocks are hidden by default. In some cases (e.g. when defining functions that should be available in all doctest groups) it would be convenient to be able to make the code visible.

Adding a show option to the testsetup directive would be one way of achieving this.

Example:

.. testsetup:: *
   :show:

   def connect():
      ...
      
 .. testcode:: group1

    conn = connect()