[Sitemaps] support for alternate links

[sebastian-nagel]

Some extensions of the sitemap protocol allow alternate links, e.g.

• links to pages for different languages or locales (spec), example from a Belgian site:

<?xml version="1.0" encoding="UTF-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:xhtml="http://www.w3.org/1999/xhtml"
        xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
  <url>
    <loc>http://www.i-run.be/electronique/</loc>
    <xhtml:link rel="alternate" hreflang="fr-FR" href="http://www.i-run.fr/electronique/"/>
    <xhtml:link rel="alternate" hreflang="fr-BE" href="http://www.i-run.be/electronique/"/>
    <xhtml:link rel="alternate" media="only screen and (max-width: 640px)" href="http://m.i-run.be/electronique/"/>
  </url>
 ...

• or media types (see above)
• or app indexing sitemaps (example):

<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9"
        xmlns:xhtml="http://www.w3.org/1999/xhtml">
   <url>
      <loc>https://bnc.lt/GC8j/s/d2ceaafba1faa604ed6046de6ec5c2dae2d6531bc0a91685aa80bace9218d369</loc>
      <lastmod>2016-06-13</lastmod>
      <xhtml:link
                rel="alternate"
                href="android-app://com.ebay.vivanuncios.mx/vivanunciosmx/search?locationid=1013&keyword=traspaso papeleria&link_click_id=s-d2ceaafba1faa604ed6046de6ec5c2dae2d6531bc0a91685aa80bace9218d369"
                />
      <xhtml:link
                rel="alternate"
                href="ios-app://872789604/vivanunciosmx/search?locationid=1013&keyword=traspaso papeleria&link_click_id=s-d2ceaafba1faa604ed6046de6ec5c2dae2d6531bc0a91685aa80bace9218d369"
                />
      </url>
      ...

Note: alternate links (if syntactically correct) do not break the sitemap parser, they are simply ignored same as additions by other extensions (image, video, news sitemaps).