Pass positional arguments into shell methods.

Much like controllers, shells now get positional arguments as method
parameters. This makes it easier to work with positional arguments, as
you don't need to use $this->args.

Refs #3325
Update bake tasks to use method arguments over $args property.

Use the new positional arguments feature to clean up bake a bit.
Merge pull request #3350 from cakephp/3.0-shell-pos-args

3.0 Pass positional shell arguments to Shell class