Allow to add raw LaTeX in sidebar via metadata

The `latex.before-metadata` value allows to add content before the
metadata in the margin column. This is helpful as an escape hatch, e.g.,
when the sidebar needs to be modified to prevent it from overflowing the
page.

E.g.:

    latex:
      before-sidebar: '\vspace{-3cm}'

Author: tarleb

data/filters/parse-latex.lua

@@ -7,7 +7,11 @@ function Pandoc (doc)
     return nil  -- do nothing
   end
 
-  return doc:walk {
+  -- The `latex` metadata field is special, raw LaTeX is always allowed in
+  -- there.
+  local latex = doc.meta.latex
+
+  doc = doc:walk {
     RawBlock = function (raw)
       if raw.format:match 'tex' then
         return pandoc.read(
@@ -29,4 +33,8 @@ function Pandoc (doc)
       end
     end
   }
+
+  -- Restore the `latex` metadata field.
+  doc.meta.latex = latex
+  return doc
 end

data/templates/default.latex

@@ -472,6 +472,9 @@ $endif$
 \marginpar{
 
   \begin{flushleft}
+$if(latex.before-metadata)$
+  ${latex.before-metadata}
+$endif$
   %\hrule
   \sffamily\small