[Feature request] site block workers

It would be nice to define these inside a `php_server` block.

```Caddyfile
one.example.com {
root /path/to/app/public
php_server {
root /path/to/app/public
env APP_ENV one
worker worker.php # inherits APP_ENV one
queue_worker queue_worker.php # inherits APP_ENV one
}
}
```

Which also means that the global directive's name shouldn't be prefixed with `m#`.