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

archetypes/posts.md

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