Adds a wpseo_news_sitemap_content filter to append custom content to the XML sitemap. Props to @wccoder.
This PR introduces a new way of retrieving translations for Yoast News SEO, by utilizing the TranslationPress service. Instead of having to ship all translations with every release, we can now load the translations on a per-install basis, tailored to the user's setup. This means smaller plugin releases and less bloat on the user's server.