Duck punch admin loader

This will let us easily build our replacement for press-this.php without loading any files directly out of the plugins folder, as if we were to include `wp-admin/admin.php` from a file loaded in the plugins folder, we would get an error something like

```
Notice: Undefined offset: 1 in /srv/www/wordpress-develop/src/wp-includes/vars.php on line 31
```

And besides, we likely shouldn't try loading executable files directly from the plugins folder if it can be helped.