Let the name-value separator be passed through -s

This allows tags with an equals sign in them to be used unambiguously.
Make vinbt respect $NORBERT

Respect the NORBERT variable even if the user is trying to pass
arguments to it (e.g. NORBERT="./norbert.py -s .:")
Change default separator

Change the name separator from ',' to '/'. This introduces
breakage again much like in commit f34ec25e, but slash looks much
better and I'd rather change it now than be stuck with commas
forever.
Update documentation, --help output

Includes several factual corrections and clarifications.