Implement indexing options

[slackero]

It's necessary to have more control over robots meta tag and how content should be indexed or not. Google has good explanations about this.

The solution should be as universal as possible and not specific to Google only – so every serious crawler can read and follow indexing options.

Article and Structure level:

• noindex
• nofollow
• noarchive
• nosnippet / max-snippet
• noimageindex / max-image-preview
• unavailable_after

Content Part:

• data-nosnippet