Pass values from config files to subcommands via $assoc_args

If your `wp-cli.yml` file looks like this:

```yaml
core install:
  title: Test Site
  url: example.com
  admin_user: admin
  admin_password: 1
  admin_email: admin@example.com
  extra: [foo, bar]
```

then you can run `wp core install` without any parameters.

Additionally, `$assoc_args['extra']` will be set to `array('foo', 'bar')`, even if `--extra` is not defined as a runtime parameter in the command docblock.

Fixes #520