Integration: Yoast SEO

[tangrufus]

Purge Yoast SEO XML sitemap urls also

• Get all Yoast SEO XML sitemap urls
• Add xml urls to purge filter

ref:

• https://github.com/Yoast/wordpress-seo/blob/master/inc/sitemaps/class-sitemaps.php
• https://github.com/Yoast/wordpress-seo/blob/4.5/inc/sitemaps/interface-sitemap-provider.php#L27