Add Twitter Cards to each page (#133)

* Add Twitter Cards to each page

Hugo has an internal template for [Twitter Cards](https://gohugo.io/templates/internal/#twitter-cards). It takes necessary information from either site's or post's config thus it makes sense to add it to each page.

* Add 'description' to posts archetype

This optional field is used by Twitter Cards.

* Add example usage of Twitter Cards

* Add artspb to contributors

layouts/_default/baseof.html

@@ -9,6 +9,8 @@
     {{ with .Site.Params.description }}<meta name="description" content="{{ . }}">{{ end }}
     {{ with .Site.Params.keywords }}<meta name="keywords" content="{{ . }}">{{ end }}
 
+    {{ template "_internal/twitter_cards.html" . }}
+
     <base href="{{ .Permalink }}">
     <title>{{ block "title" . }}{{ .Site.Title }}{{ end }}</title>