Adds the ability to pass arguments to the tox command.

With it I can run a single tests over many pythons like this:

    tox -- -k test_a_specific_test

See http://tox.readthedocs.org/en/latest/example/general.html#interactively-passing-positional-arguments
Merge pull request #247 from seantis/master

Adds the ability to pass arguments to the tox command.