Implement the new fields for plain list.

src/types/greater_element.rs

@@ -13,6 +13,8 @@ pub struct PlainList<'s> {
     pub affiliated_keywords: AffiliatedKeywords<'s>,
     pub list_type: PlainListType,
     pub children: Vec<PlainListItem<'s>>,
+    pub contents: Option<&'s str>, // TODO: Can contents ever be None?
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug, Copy, Clone)]
@@ -142,11 +144,15 @@ impl<'s> StandardProperties<'s> for PlainList<'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.")
     }
 }