Implement Facebook domain verification

[fauno]

Hi! Needed this for a project :)

https://developers.facebook.com/docs/sharing/domain-verification/

[ashmaroli]

I'm on board with the proposal but hesitant nevertheless because this further affects performance.

{% if site.webmaster_verifications %}
  ...
  {% if site.webmaster_verifications.facebook %}
    ...
  {% endif %}
{% endif %}

The above block is evaluated for every page / document of a site using this plugin to return the same result.
We should think of way to optimize this..

[fauno]

I wonder if it could only be rendered on the home page... you don't normally verify other pages, but not sure what each provider does.

[parkr]

Seems fine to me!

[ashmaroli]

Thanks @fauno
@jekyllbot; merge +minor

[ashmaroli]

@jekyllbot: merge +minor