Add initial implementation of footnote definition.

src/parser/plain_list.rs

@@ -27,6 +27,7 @@ use nom::multi::many_till;
 use nom::sequence::tuple;
 
 pub fn plain_list<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s str, PlainList<'s>> {
+    // TODO: Are we handling 2 blank lines causing the end of all plain lists?
     let (mut remaining, first_item) = plain_list_item(context, input)?;
     let first_item_indentation = first_item.indentation;
     let plain_list_item_matcher = parser_with_context!(plain_list_item)(context);