Use meta-description instead of description in templates.

Since this is an attribute value, we need to prepare it
in the writer.

data/templates/default.html4

@@ -13,8 +13,8 @@ $endif$
 $if(keywords)$
   <meta name="keywords" content="$for(keywords)$$keywords$$sep$, $endfor$" />
 $endif$
-$if(description)$
-  <meta name="description" content="$description$" />
+$if(description-meta)$
+  <meta name="description" content="$description-meta$" />
 $endif$
   <title>$if(title-prefix)$$title-prefix$ – $endif$$pagetitle$</title>
   <style type="text/css">

data/templates/default.html5

@@ -13,8 +13,8 @@ $endif$
 $if(keywords)$
   <meta name="keywords" content="$for(keywords)$$keywords$$sep$, $endfor$" />
 $endif$
-$if(description)$
-  <meta name="description" content="$description$" />
+$if(description-meta)$
+  <meta name="description" content="$description-meta$" />
 $endif$
   <title>$if(title-prefix)$$title-prefix$ – $endif$$pagetitle$</title>
   <style>

src/Text/Pandoc/Writers/HTML.hs

@@ -254,6 +254,8 @@ pandocToHtml opts (Pandoc meta blocks) = do
   let stringifyHTML = escapeStringForXML . stringify
   let authsMeta = map stringifyHTML $ docAuthors meta
   let dateMeta  = stringifyHTML $ docDate meta
+  let descriptionMeta = escapeStringForXML $
+                          lookupMetaString "description" meta
   slideVariant <- gets stSlideVariant
   let sects = adjustNumbers opts $
               makeSections (writerNumberSections opts) Nothing $
@@ -352,6 +354,7 @@ pandocToHtml opts (Pandoc meta blocks) = do
                   defField "author-meta" authsMeta .
                   maybe id (defField "date-meta")
                     (normalizeDate dateMeta) .
+                  defField "description-meta" descriptionMeta .
                   defField "pagetitle"
                       (stringifyHTML . docTitle $ meta) .
                   defField "idprefix" (writerIdentifierPrefix opts) .