Initial structure for adding support for comments.

src/compare/diff.rs

@@ -216,6 +216,7 @@ fn compare_element<'s>(
         Element::PlainList(obj) => compare_plain_list(source, emacs, obj),
         Element::GreaterBlock(obj) => compare_greater_block(source, emacs, obj),
         Element::FootnoteDefinition(obj) => compare_footnote_definition(source, emacs, obj),
+        Element::Comment(obj) => todo!(),
     }
 }