Experiment runner should take an optional argv argument

[dmoisset]

It's customary when providing APIs for runners to provide an optional argv argument to use instead of sys.argv. This allows building custom runners more easily or overriding/defaulting arguments. It also makes the runner argumetn parsing easier to unit test

As an example of this API pattern in other places, you can take a look at https://github.com/docopt/docopt#api or https://docs.python.org/2/library/unittest.html#unittest.main