tox commands now have {posargs} as argument

When invoking an environement, one might want to pass extra argument to
the command. That is done in tox by invoking an env and passing the
extra arguments after '--' which are then available as '{posargs}'.

Examples:

  # Reports flake8 error statistics
  tox -eflake8 -- --statistics

  # Only run test_util.py tests, printing a line per test:
  tox -epy27 -- --verbose  git/test/test_util.py