Move trailing whitespace parsing to a separate element.

I still need to parse the line break at the end of elements.

src/parser/plain_list.rs

@@ -39,7 +39,6 @@ pub fn plain_list<'r, 's>(context: Context<'r, 's>, input: &'s str) -> Res<&'s s
         }),
         exit_matcher,
     )(remaining)?;
-    let (remaining, _trailing_whitespace) = trailing_whitespace(remaining)?;
     let source = get_consumed(input, remaining);
     children.insert(0, first_item);
     Ok((remaining, PlainList { source, children }))