Support meta robots tag.

[PMExtra]

PR Checklist

• The commit message follows guidelines for NexT.
• Tests for the changes was maked (for bug fixes / features).
  • Muse | Mist have been tested.
  • Pisces | Gemini have been tested.
• Docs in NexT website have been added / updated (for features).

PR Type

• Feature.

What is the new behavior?

Robots meta directives (sometimes called "meta tags") are pieces of code that provide crawlers instructions for how to crawl or index web page content. (For more details: https://moz.com/learn/seo/robots-meta-directives)

You can tell the search engines to don't index the page by setting the value noindex to true in Front-matter.
You can also specify a certain value of robots directive by setting the robots in Front-matter.

• Link to demo site with this changes: https://blog.jubeat.net/cv It's set noindex: true

How to use?

In your any custom_page.md:

title: custom_name
date: 2014-12-22 12:39:04
robots: noimageindex, nofollow, nosnippet
noindex: true # This will be ignored if you've set the robots.
---

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[PMExtra]

I've improved the docs of this. Please let me know if anything I can do, or accept the PR.