Truncate the description value to 280 chars

Currently the value of the `description`, `og: description`
and `twitter: description` tags includes the whole text of the post.

Which means that, on post pages, the text of the post is included *four*
times in the HTML document – adding to the page weight needlessly.

This commit ensures the description string is truncated to a sane value.

lib/jekyll-seo-tag/drop.rb

@@ -89,7 +89,8 @@ module Jekyll
 
       def description
         @description ||= begin
-          format_string(page["description"] || page["excerpt"]) || site_description
+          value = format_string(page["description"] || page["excerpt"]) || site_description
+          filters.truncate(value, 280)
         end
       end