Improve footnote handling

- Footnotes separator rather than headline to get around i18n
- Warn on footnote redefinition
- Do not export footnote definitions at point of definition, only in the
  footnote section.
- Do not automatically exclude Footnotes section to get around possibly hiding
  other content of such a section - and i18n.
  The user has the choice of explicitly hiding the section via a :noexport:
  tag.

and some other refactoring

org/footnote.go

@@ -35,17 +35,10 @@ func (d *Document) parseFootnoteDefinition(i int, parentStop stopFn) (int, Node)
 	}
 	consumed, nodes := d.parseMany(i, stop)
 	definition := FootnoteDefinition{name, nodes, false}
-	d.Footnotes.add(name, &definition)
+	d.addFootnote(name, &definition)
 	return consumed, definition
 }
 
-func (fs *Footnotes) add(name string, definition *FootnoteDefinition) {
-	if definition != nil {
-		fs.Definitions[name] = definition
-	}
-	fs.addOrder = append(fs.addOrder, name)
-}
-
 func (fs *Footnotes) Ordered() []FootnoteDefinition {
 	m := map[string]bool{}
 	definitions, inlineDefinitions := []FootnoteDefinition{}, []FootnoteDefinition{}