Implement the new fields for horizontal rule.

src/parser/horizontal_rule.rs

@@ -38,7 +38,7 @@ where
         space0,
         alt((line_ending, eof)),
     )))(remaining)?;
-    let (remaining, _trailing_ws) =
+    let (remaining, post_blank) =
         maybe_consume_trailing_whitespace_if_not_exiting(context, remaining)?;
     let source = get_consumed(input, remaining);
     Ok((
@@ -49,6 +49,7 @@ where
                 context.get_global_settings(),
                 affiliated_keywords,
             ),
+            post_blank: post_blank.map(Into::<&str>::into),
         },
     ))
 }

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.")
     }
 }