Allow multisite networks to be initialized without editing the config file.

It's a hack, silly core, but our wp-config.php looks like this:

```
if ( ! defined( 'WP_INSTALLING' ) || ! WP_INSTALLING ) {
	define('WP_ALLOW_MULTISITE', true);
	if ( ! defined( 'WP_CLI' )
	|| ( defined( 'WP_CLI' ) && array( 'core', 'install-network' ) != WP_CLI::$runner->arguments ) ) {
		define('SUBDOMAIN_INSTALL', true);
		define('PATH_CURRENT_SITE', '/');
		define('SITE_ID_CURRENT_SITE', 1);
		define('BLOG_ID_CURRENT_SITE', 1);
		define( 'MULTISITE', true );
	}
}
```

This lets us install the project via wp-cli in two parts:

```
wp core install
wp core install-network
```

Right now though, wp-cli awkwardly adds the constants at the end of our config file