Add Twitter card meta tags

app/views/asciicasts/_discovery_tags.html.erb

@@ -9,3 +9,11 @@
 <meta property="og:image" content="<%= asciicast_url(page.asciicast, format: :png) %>" />
 <meta property="og:description" content="<%= page.short_text_description %>" />
 <meta property="og:site_name" content="asciinema.org" />
+
+<!-- Twitter card -->
+<meta name="twitter:card" content="summary_large_image" />
+<meta name="twitter:site" content="@asciinema" />
+<meta name="twitter:title" content="<%= page.asciicast_title %>" />
+<meta name="twitter:url" content="<%= asciicast_url(page.asciicast) %>" />
+<meta name="twitter:description" content="<%= page.short_text_description %>" />
+<meta name="twitter:image" content="<%= asciicast_url(page.asciicast, format: :png) %>" />