Generate all files needed for running plugin unit tests

Currently, when you run `wp scaffold plugin foo` it only creates the `foo.php` php file, with the plugin header.

It would be nice if it would also take care of creating all the files needed for running unit tests via phpunit.

Would also be nice to have `wp scaffold plugin-tests <plugin>` to add the test files for an existing plugin.

Going to base this on the files I use for [wp-posts-to-posts](http://github.com/scribu/wp-posts-to-posts).

Another concern is how to generate the `unit-tests-config.php` file; it should use the WP install from which wp-cli is invoked.