Add optional support for image indexing

[vezaynk]

Spec: https://support.google.com/webmasters/answer/178636

Example:

<?xml version="1.0" encoding="UTF-8"?> <urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:image="http://www.google.com/schemas/sitemap-image/1.1">
    <url>
      <loc>http://example.com/sample.html</loc> 
      <image:image> 
        <image:loc>http://example.com/image.jpg</image:loc>
      </image:image> 
      <image:image>
        <image:loc>http://example.com/photo.jpg</image:loc>
      </image:image>
    </url>
</urlset> 

Google's sitemap image spec needs to be included.

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[vezaynk]

By lack of easier implementation, an img_index function will be called from within the scan_url function after failing the header check if the option is enabled.

[vezaynk]

Personal objective: Going to try to do it over the weekend

[vezaynk]

I have misjudged the extent of the effort. This opens it's own can of worms.

1. Scan hrefs and imgs
2. Identifying images
3. Keeping track of context

This is not something I can do in a weekend. I am tempted to mark this as out-of-scope but it looks like a fun feature to try to implement. While I will never officially support it, I might probably do it.

With that said, PRs are welcome if anybody wants to do this themselves in the meantime!

[ghost]

Licenses are important for image sitemaps as there are no other feasible methods for communicating image licenses to search engines. If they are site wide, there could be a command line option for giving the license (an URL to actual license), like for example:
--license http://creativecommons.org/publicdomain/zero/1.0/

Which would output to sitemap.xml inside <image:loc>:

<image:license>http://creativecommons.org/publicdomain/zero/1.0/</image:license>

There are also some other tags in image sitemaps that could be read from the html if they are present including:

• <image:title> - TITLE and/or ALT tag
• <image:caption> - FIGCAPTION, TITLE and/or ALT

Video sitemaps are not very different from image sitemaps either, but here are a few more obligatory tags:

• Title
• Description
• Play page URL (web page URL)
• Thumbnail URL
• Video file URL

These could be crawled from the html, or if not present populated with placeholders.

After the image crawling is working, I am happy to offer the project an online environment I have already coded where people can generate image sitemaps.

I will take a look at your code later and see if I can throw in something more tangible than just ideas and testing.