Fixed titleblock in markdown, plain templates.

Now it is just a single string. Previously separate
title, author, and date variables were used, but this didn't
allow different kinds of title blocks.

default.markdown

@@ -1,7 +1,5 @@
 $if(titleblock)$
-% $title$
-% $for(author)$$author$$sep$; $endfor$
-% $date$
+$titleblock$
 
 $endif$
 $for(header-includes)$

default.plain

@@ -1,7 +1,5 @@
 $if(titleblock)$
-$title$
-$for(author)$$author$$sep$; $endfor$
-$date$
+$titleblock$
 
 $endif$
 $for(header-includes)$