Add support for arg() and option()

[prudloff-insite]

To be able to do things like that:

$this->taskDrushStack()->drush('sget')->arg('system.maintenance_mode');

[prudloff-insite]

OK so I just noticed that it works but only if these are called before drush():

$this->taskDrushStack()->arg('system.maintenance_mode')->drush('sget');

It is a bit confusing.