Exclude author from sitemap

Fix for issue: #972

- Individual users can be excluded from sitemaps (checkbox addition in user edit page)
- User roles can be excluded from sitemaps (on the sitemap settings page)
And with the defaults being in place this should check whether the $options[$target key] === true. No need to check for array_key_exists.
You could actually removed the whole if clause and just change this to :
```php
$exclude_user = $options[$target key];
```