Record the list item indent depth in the context tree.

src/parser/parser_context.rs

@@ -87,6 +87,7 @@ impl<'r, 's> ContextTree<'r, 's> {
                 ContextElement::PreviousElementNode(_) => {}
                 ContextElement::StartOfParagraph => {}
                 ContextElement::Context(_) => {}
+                ContextElement::ListItem(_) => {}
             };
         }
         // TODO: Make this a specific error instead of just a generic MyError
@@ -99,6 +100,7 @@ pub enum ContextElement<'r, 's> {
     ExitMatcherNode(ExitMatcherNode<'r>),
     PreviousElementNode(PreviousElementNode<'s>),
     Context(&'r str),
+    ListItem(usize),
     StartOfParagraph,
 }