The Cmd class provides a simple framework for writing
line-oriented command interpreters. These are often useful for
test harnesses, administrative tools, and prototypes that will
later be wrapped in a more sophisticated interface.
A Cmd instance or subclass instance is a line-oriented
interpreter framework. There is no good reason to instantiate
Cmd itself; rather, it's useful as a superclass of an
interpreter class you define yourself in order to inherit
Cmd's methods and encapsulate action methods.
The optional argument is the readline name of a completion
key; it defaults to Tab. If completekey is not None
and readline is available, command completion is done
automatically.