Implement the new fields for comment.

src/types/lesser_element.rs

@@ -237,11 +237,11 @@ impl<'s> StandardProperties<'s> for Comment<'s> {
     }
 
     fn get_contents<'b>(&'b self) -> Option<&'s str> {
-        todo!()
+        None
     }
 
     fn get_post_blank(&self) -> PostBlank {
-        todo!()
+        0
     }
 }