Allow arguments to --cmd to contain tabs or multiple spaces

[MarkBaggett]

What do you think of allowing split to separate the --cmd argument on any white space instead of specifically one space?

After a few days of fighting with the following command line syntax:

butterfly.server.py --host=0.0.0.0 --port=57575 --cmd='/usr/bin/docker run --rm -it --mount type=bind,source=/home/mark/app,target=/app,readonly myimage /bin/bash'

I was able to determine that it was breaking because I had more than one space between "readonly" and "myimage". This syntax works just fine:

butterfly.server.py --host=0.0.0.0 --port=57575 --cmd='/usr/bin/docker run --rm -it --mount type=bind,source=/home/mark/app,target=/app,readonly myimage /bin/bash'

Such a subtle difference is easily unnoticed by the user.