Update to work better with Yoast SEO

[wpscholar]

At the moment, this plugin only adds a noindex robots meta tag. However, the Yoast SEO plugin also outputs a robots meta tag.

If Yoast SEO isn't blocking indexing for a page where a user of our plugin has checked the "Block search engines" box, we should override the Yoast SEO setting using the wpseo_robots hook:

add_filter(
	'wpseo_robots',
	function ( $value ) {
		if ( /* Perform check */ ) ) {
			return 'noindex,follow';
		}

		return $value;
	}
);

We should, however, respect the Yoast SEO nofollow rule if set. At the moment, we default to follow.