layouts: Make featured posts configurable

For now it gets the first three posts with "featured: true" in their
frontmatter and displays them in a random order (so no control over
the placement of large feature or two smaller features).

TODO: gracefully handle the case where there are no pages with the
"featured: true" frontmatter so new users don't see a super weird
and broken front page.

layouts/_default/home.html

@@ -39,7 +39,7 @@
       </h3>
 
 {{ $truncate := default true .Site.Params.truncate }}
-{{ $paginator := .Paginate (where .Site.RegularPages "Section" "in" .Site.Params.mainSections) }}
+{{ $paginator := .Paginate (where .Site.RegularPages "Params.featured" "ne" true) }}
 {{ range $paginator.Pages }}
 {{ if $truncate }}
 {{ .Render "summary" }}