Added support for the subject metadata variable to PDF. (#5289)

MANUAL.txt

@@ -1394,7 +1394,7 @@ Metadata variables
     and AsciiDoc metadata; repeat as for `author`, above
 
 `subject`
-:   document subject, included in ODT, docx and pptx metadata
+:   document subject, included in ODT, PDF, docx and pptx metadata
 
 `description`
 :   document description, included in ODT, docx and pptx metadata. Some

data/templates/default.latex

@@ -205,6 +205,9 @@ $endif$
 $if(author-meta)$
   pdfauthor={$author-meta$},
 $endif$
+$if(subject)$
+  pdfsubject={$subject$},
+$endif$
 $if(keywords)$
   pdfkeywords={$for(keywords)$$keywords$$sep$, $endfor$},
 $endif$