Implement the parser for anonymous footnotes.

src/parser/object.rs

@@ -121,7 +121,7 @@ pub struct ExportSnippet<'s> {
 #[derive(Debug, PartialEq)]
 pub struct FootnoteReference<'s> {
     pub source: &'s str,
-    pub label: &'s str,
+    pub label: Option<&'s str>,
     pub definition: Vec<Object<'s>>,
 }