Add some OG image

See #443

Author: swissspidy

_includes/header.html

@@ -13,7 +13,21 @@
 		<link rel="alternate" type="application/atom+xml" href="/atom.xml" title="Atom Feed">
 
 		<title>{{ page.title }} | WP-CLI</title>
-		<meta name="description" content="{{ page.description }}">
+		<meta name="description" content="{{ page.description | default: 'The command line interface for WordPress' }}">
+		<meta property="og:type" content="website" />
+		<meta property="og:title" content="{{ page.title | default: 'WP-CLI' }}" />
+		<meta property="og:url" content="{{ site.url }}{{ page.url }}" />
+		<meta property="og:description" content="{{ page.description | default: 'The command line interface for WordPress' }}" />
+		<meta property="og:site_name" content="WP-CLI" />
+		<meta property="og:image" content="{{ site.url }}/assets/img/wp-cli-og-image.png" />
+		<meta property="og:image:alt" content="WP-CLI Logo" />
+		<meta property="og:locale" content="en_US" />
+		<meta name="twitter:site" content="@wpcli" />
+		<meta name="twitter:title" content="{{ page.title | default: 'WP-CLI' }}" />
+		<meta name="twitter:description" content="{{ page.description | default: 'The command line interface for WordPress' }}" />
+		<meta name="twitter:image" content="{{ site.url }}/assets/img/wp-cli-og-image.png" />
+		<meta name="twitter:card" content="summary_large_image" />
+
 
 		{% for lang in site.languages %}<link rel="alternate" hreflang="{{lang.iso-alpha2}}" href="{{site.url}}{{lang.path}}" />
 		{% endfor %}