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

CONTRIBUTORS.md

@@ -26,3 +26,4 @@
 - [Martin Kiesel](https://github.com/Kyslik)
 - [John Tobin](https://www.johntobin.ie/)
 - [Thomas Nys](https://thomasnys.com)
+- [Artem Khvastunov](https://artspb.me)

archetypes/posts.md

@@ -2,6 +2,7 @@
 draft = true
 date = {{ .Date }}
 title = ""
+description = ""
 slug = "" 
 tags = []
 categories = []

exampleSite/content/posts/migrate-from-jekyll.md

@@ -1,6 +1,7 @@
 +++ 
 date = "2014-03-10"
 title = "Migrate to Hugo from Jekyll"
+description = "The post explains how to migrate from from Jekyll to Hugo."
 +++
 
 Table of Contents

exampleSite/content/posts/theme-demo.md

@@ -1,6 +1,8 @@
 +++
 date = "2017-01-08"
 title = "Theme Demo"
+description = "The post demonstrates features of the coder theme."
+images = ["/images/N90.jpg"]
 math = "true"
 
 +++

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>