Pyxargs

- Made same mistake when copying & pasting help to readme

- Update argparse, the python documentation for nargs="+" was unclear and "Additionally, an error message will be generated if there wasn’t at least one command-line argument present" only applies to positional arguments or if the flag is invoked

- Update readme (run with just pyxargs when installed with pip, not python pyxargs.py)

- add interactive mode
- update documentation
- feels pretty stable and complete

run user input code in its own namespace