Continued work on item parser.

Still needs to:

1. Do context things
2. Fix the double-space after the tag separator issue
3. Add support for indentation
4. Write item_end
5. Write plain_list

src/parser/token.rs

@@ -111,9 +111,9 @@ impl<'a> Source<'a> for PlainList<'a> {
 pub struct ListItem<'a> {
     pub source: &'a str,
     pub bullet: &'a str,
-    pub counter_set: &'a str,
-    pub check_box: &'a str,
-    pub item_tag: &'a str,
+    pub counter_set: Option<&'a str>,
+    pub check_box: Option<&'a str>,
+    pub item_tag: Option<&'a str>,
     pub contents: Vec<TextElement<'a>>,
 }