Implement the new fields for footnote definitions.

src/types/greater_element.rs

@@ -82,6 +82,8 @@ pub struct DynamicBlock<'s> {
 #[derive(Debug)]
 pub struct FootnoteDefinition<'s> {
     pub source: &'s str,
+    pub contents: Option<&'s str>,
+    pub post_blank: Option<&'s str>,
     pub affiliated_keywords: AffiliatedKeywords<'s>,
     pub label: &'s str,
     pub children: Vec<Element<'s>>,
@@ -218,11 +220,15 @@ impl<'s> StandardProperties<'s> for FootnoteDefinition<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        self.contents
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        self.post_blank
+            .map(|text| text.lines().count())
+            .unwrap_or(0)
+            .try_into()
+            .expect("Too much post-blank to fit into a PostBlank.")
     }
 }