User argparse to override anything from defaults, RC files or
environmental variables.

Also, add a mambupy_rcfile argument to override the $HOME/.mambupy.rc
and /etc/mambupy.rc option to point to any RC file you wish to use.

TECHDEBT: if you use command line arguments on your own programs, you
MUST be aware of catching 'unknown' arguments you send to MambuPy,
since they will be recognized by mambuconfig but not by your own
programs.

For example, using argparse library, you must use parse_known_args()
instead of parse_args() to parse command line arguments.