Allow plugins/themes to modify the generated XML

[dana-ross]

I'd like to start using msm-sitemap for a site's main sitemaps, but keep using Yoast's News Sitemap plugin until there's an alternative for generating news sitemaps. So, I need a way to add this additional sitemap URL to the end of the index.

Yoast's SEO plugin has a filter to accomplish this, which their News Sitemap plugin hooks into:

        /**
         * Filter to append sitemaps to the index.
         *
         * @param string $index String to append to sitemaps index, defaults to empty.
         */
        $xml .= apply_filters( 'wpseo_sitemap_index', '' );

I think it would make sense to add a similar filter to this plugin.