Related Posts: Add filter for `_enabled_for_request()`

Allow the return value of function to be filtered by
`jetpack_relatedposts_filter_enabled_for_request` this will allow related posts
to be shown on pages as well.

Example:

```php
function jetpackme_enable_for_pages( $enabled ) {
	if ( is_page() )
		$enabled = true;

	return $enabled;
}
add_filter( 'jetpack_relatedposts_filter_enabled_for_request', 'jetpackme_enable_for_pages');
```

Please note by default related posts will only return related content of the
same `post_type` so doing the above will only return related pages not posts to
adjust this see the `jetpack_relatedposts_filter_post_type` filter.

Resolves #1307