card_type param option is added for Twitter cards

Now in config file, a desired parameter, which are listed in Twitter documentation put in README,
can be set for card content

README.md

@@ -214,6 +214,10 @@ paginate = 5
   # include @
   creator = ""
   site = ""
+  # set Twitter card type for Twitter cards
+  # see https://developer.twitter.com/en/docs/twitter-for-websites/cards/guides/getting-started#started
+  # default value is "summary"
+  card_type = "summary"
 
 [languages]
   [languages.en]

layouts/partials/head.html

@@ -37,7 +37,11 @@
 {{ end }}
 
 <!-- Twitter Card -->
-<meta name="twitter:card" content="summary" />
+{{ if (isset $.Site.Params.Twitter "content") }}
+  <meta name="twitter:card" content="{{ $.Site.Params.Twitter.content }}" />
+{{ else }}
+  <meta name="twitter:card" content="summary" />
+{{ end }}
 {{ if (isset $.Site.Params "twitter") }}
   {{ if (isset $.Site.Params.Twitter "site") }}
     <meta name="twitter:site" content="{{ $.Site.Params.Twitter.site }}" />