Invoke pew with a double dash separator ("--"), to make it clear that
the virtualenv name is a positional argument. Since the virtualenv
name includes all or at least the beginning of the directory name,
trying to create a virtualenv in a directory with a leading dash in
its name will cause pew to erroneously parse the virtualenv name as an
optional argument. Adding the separator causes the virtualenv name to
be parsed correctly.

Fixes pypa#439