noindex support for small results

- Added noindex_pages_with_fewer_than_n_results param
- If results is less than the noindex_pages_with_fewer_than_n_results param we insert noindex meta
	- `<meta name="robots" content="noindex, follow" />`

https://github.com/GigaOM/gigaom/issues/5221
I would very much prefer to keep the noindexing behavior consolidated into the same class, rather than split between two different classes, each with their own supporting functions.

Perhaps you investigated and had good reason not to implement as suggested [in the ticket](https://github.com/GigaOM/gigaom/issues/5221), though?

> Rename [`scriblio`'s `_count_found_posts()` method](https://github.com/misterbisson/scriblio/blob/master/plugin/class-facets.php#L255) to `template_redirect()` and inspect the count of found posts _and_ the value of `$this->config['noindex_pages_with_fewer_than_n_results']`. If the config entry is set and the value of found posts is smaller than the integer in the config file, then `add_action( 'wp_head', array( $this, 'wp_head_noindex' ) );`