Full support for sitemap extensions and namespaces

[sebastian-nagel]

From sitemaps only news sitemaps are accepted as seed source. However,

• information from extensions is not used (cf. news:publication_date #18)
• any sitemap which declares the news namespace is considered to be a news sitemap. But there are also image sitemaps which declare the news namespace:
  • the image URLs should not be extracted as news seeds
  • ev. also items not marked as news should be skipped

    <?xml version="1.0" encoding="UTF-8" standalone="yes"?>
    <urlset xmlns="http://www.sitemadps.org/schemas/sitemap/0.9" xmlns:image="http://www.google.com/schemas/sitemap-image/1.1" xmlns:news="http://www.google.com/schemas/sitemap-news/0.9">
    <url>
    <loc>...</loc>
    <lastmod>2018-03-18T21:44:50-07:00</lastmod>
    <image:image>
    <image:loc>...</image:loc>
    <image:title>...

Cf. crawler-commons/crawler-commons#162, crawler-commons/crawler-commons#174.