Introduce WP_CLI::load_wordpress()

Having a function that you can call would simplify the logic quite a bit, particularly for `wp help`:

```php
class Help_Command {

	function __invoke( $args ) {
		$command = find_subcommand( $args );
		if ( !$command ) {
			WP_CLI::load_wordpress();
		}

		if ( !$command ) {
			WP_CLI::error( "Can't find {$args[0]} command." );
		}

		// do stuff with $command
	}
}
```

Same thing with `core install`:

```php
function install( ... ) {
  define( 'WP_INSTALLING', true );
  // set URL
  WP_CLI::load_wordpress();
  // perform install
}
```

It would also help with early commands: #453.

Can copy the logic from the official unit test framework, which needs to solve the same problem.