Support multiple video "tag"

Currently, if we create a sitemap like this :

sitemap.add({
        // [ ... ]
        video: [{
          // [ ... ]
          tag: ['one', 'two'], // assuming that multiple video tags are passed as an array
          // [ ... ]
        }]
      });

the actual sitemap output is :

// [ ... ]
<video:tag>one,two</video:tag>
// [ ... ]

but should be this, according to Google :

// [ ... ]
<video:tag>one</video:tag>
<video:tag>two</video:tag>
// [ ... ]

Which means this code must loop through the tag and insert a new <video:tag> for each tag.

Anyway, thanks for this lib, it has been very helpful to me 👍

[Ilshidur]

I opened a PR : https://github.com/ekalinin/sitemap.js/pull/172

[vtenq]

cool, thank you a lot!