Improve Facebook share button

[rgaiacs]

Facebook share button uses Open Graph protocol. We could improve the share button by including

  <meta property="og:site_name" content="{{ site.data.setting.title }}" />
  <meta property="og:title" content="{{ page.title }}" />
  <meta property="og:type" content="website" />
  <meta property="og:url" content="{{ site.url }}{{ page.url }}" />
  <meta property="og:image" content="{{ site.url }}/images/{{ page.image.feature }}" />

on _includes/head.html and changing the share button to

<a href="https://www.facebook.com/sharer/sharer.php?u={{ site.url }}{{ page.url }}" target="_blank"><i class="fa fa-facebook" aria-hidden="true"></i></a>

on _layouts/post.html.