An improved cmd.py for Writing Multi-command Scripts and Shells

`cmdln.py` is an extension of Python's default `cmd.py` module that
provides "a simple framework for writing line-oriented command
interpreters".  The idea (with both cmd.py and cmdln.py) is to be able
to quickly build multi-sub-command tools (think cvs or svn) and/or
simple interactive shells (think gdb or pdb).  Cmdln's extensions make
it more natural to write sub-commands, integrate optparse for simple
option processing, and make having good command documentation easier.