added _display_files as separate method
added do_update and do_export and helper methods

These methods were added to allow the transfer of settings and state
back from the interpreter.  Returning the record *does* quit the
interpreter but does not pass the dictionary it seems.
installed parent interpreters Jack and Fetch

Exporting the fetch record through references to the child record.
Next I need proper __init__ methods and super for old-style classes.
added __init__ methods and linked subshells

Cmd is written with the old-style classes in Py27, so the usual
super methods do not work.

Instead use , Base.__init__(self).

Now prompts are instance attributes rather than class, and build from
parent prompts.
The Fetch record is linked from parent shells and passed back to Fetch.