Implement the new fields for horizontal rule.

src/types/lesser_element.rs

@@ -155,6 +155,7 @@ pub struct FixedWidthArea<'s> {
 pub struct HorizontalRule<'s> {
     pub source: &'s str,
     pub affiliated_keywords: AffiliatedKeywords<'s>,
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug)]
@@ -422,11 +423,15 @@ impl<'s> StandardProperties<'s> for HorizontalRule<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        None
     }
 
     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.")
     }
 }