layouts: Add basic support for OpenGraph and Twitter card metadata

Uses basic data like title, description, author, and date that we
are using with existing vanilla meta tags, but extends them to OG
and Twitter Card tags. See [0] and [1].

For the Twitter summary cards specifically, you can optionally add
attribution for your username by adding something like this to your
configuration:

  [params.social]
    twitter_username = "@mralanorth"

... and for posts with images, you can specify an "image" in the
post's front matter like so:

  image = "/2016/09/IMG_20160916_174409.jpg"

And then the theme will opt to use Twitter's "large" summary card.

[0] http://ogp.me/
[1] https://dev.twitter.com/cards/getting-started

layouts/_default/baseof.html

@@ -7,12 +7,7 @@
     <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
     {{ "<!-- The above 3 meta tags *must* come first in the head; any other head content must come *after* these tags -->" | safeHTML }}
 
-    <meta name="description" content="{{ .Params.description | default .Site.Params.description }}">
+    {{ partial "metadata.html" . }}
-    <meta name="author" content="{{ .Params.author | default .Site.Params.author }}">
-
-    {{ if .Site.Params.google_verify_meta }}<meta name="google-site-verification" content="{{ .Site.Params.google_verify_meta }}">{{ end }}
-
-    {{ .Hugo.Generator }}
 
     <base href="{{ .Site.BaseURL }}">
     <link rel="canonical" href="{{ .Permalink }}">

layouts/partials/metadata.html

@@ -0,0 +1,36 @@
+{{ $title := .Title | default .Site.Title }}
+{{ $description := .Params.description | default .Site.Params.description }}
+{{ $author := .Params.author | default .Site.Params.author }}
+
+    <meta name="description" content="{{ $description }}">
+    <meta name="author" content="{{ $author }}">
+
+    {{ "<!-- OpenGraph Metadata: http://ogp.me/ -->" | safeHTML }}
+    <meta property="og:title" content="{{ $title }}">
+    <meta property="og:description" content="{{ $description }}">
+
+    {{ if eq .Section "post" }}
+    <meta property="og:type" content="article">
+    <meta property="article:published_time" content="{{ .Date.Format "2006-01-02T15:04:05Z07:00" }}">
+    <meta property="article:author" content="{{ $author }}">
+      {{ if .Params.image }}
+      <meta property="og:image" content="{{ .Params.image | absURL }}">
+      {{ end }}
+    {{ else }}
+    <meta property="og:type" content="blog">
+    {{ end }}
+
+    <meta property="og:url" content="{{ .Permalink }}">
+
+    {{ "<!-- Metadata for Twitter: https://dev.twitter.com/cards/markup -->" | safeHTML }}
+    {{ if and (eq .Section "post") (.Params.image) }}
+    <meta property="twitter:card" content="summary_large_image">
+    <meta property="twitter:image" content="{{ .Params.image | absURL }}">
+    {{ else }}
+    <meta property="twitter:card" content="summary">
+    {{ end }}
+    {{ if .Site.Params.social.twitter_username}}<meta property="twitter:creator" content="{{ .Site.Params.social.twitter_username }}">{{ end }}
+    <meta property="twitter:title" content="{{ $title }}">
+    <meta property="twitter:description" content="{{ $description }}">
+
+    {{ .Hugo.Generator }}