Support leading whitespace for list items.

src/parser/token.rs

@@ -110,6 +110,7 @@ impl<'a> Source<'a> for PlainList<'a> {
 #[derive(Debug)]
 pub struct ListItem<'a> {
     pub source: &'a str,
+    pub leading_whitespace: &'a str,
     pub bullet: &'a str,
     pub counter_set: Option<&'a str>,
     pub check_box: Option<&'a str>,