Improve meta description

Rather than use the same description on every page, prefer the post
summary if it's available and only use .Params.description on the root
page.

This mimics the behavior of the [opengraph internal
template](https://github.com/gohugoio/hugo/blob/master/tpl/tplimpl/embedded/templates/opengraph.html#L2)
for the `og:description` tag.

layouts/partials/header.html

@@ -14,9 +14,7 @@
 	{{- end -}}
 
 	<meta name="viewport" content="width=device-width, initial-scale=1">
-	{{- if isset .Site.Params "description" }}
-	<meta name="description" content="{{ .Site.Params.description }}" />
-	{{- end }}
+	<meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{if .IsPage}}{{ .Summary }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}{{ end }}" />
 	<meta property="og:image" content="{{ .Site.Params.og_image }}"/>
 	{{ with .OutputFormats.Get "rss" -}}
 	{{ printf `<link rel="%s" type="%s" href="%s" title="%s" />` .Rel .MediaType.Type .Permalink $.Site.Title | safeHTML }}