Implement the new fields for planning.

src/types/lesser_element.rs

@@ -141,6 +141,7 @@ pub struct Planning<'s> {
     pub scheduled: Option<Timestamp<'s>>,
     pub deadline: Option<Timestamp<'s>>,
     pub closed: Option<Timestamp<'s>>,
+    pub post_blank: Option<&'s str>,
 }
 
 #[derive(Debug)]
@@ -391,11 +392,15 @@ impl<'s> StandardProperties<'s> for Planning<'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.")
     }
 }