Add a "--version" argument to the command line interface

In order to keep things consistent and easily maintainable, we import
`__version__` from the `__init__.  I usually just `import package_name`
and then I use `version=package_name.__version__`.

This time, since `cookiecutter` is a function in the same namespace and to avoid
any ambiguity, we import the package as `cookiecutter_package`.