add tests for html generation, refactor lists

src/ast.rs

@@ -31,6 +31,6 @@ pub enum Block {
     Code { content: String, lang: String },
     Quote { inner: Box<Block> },
     Paragraph { inner: Vec<Inline> },
-    UnorderedList { items: Vec<Block> },
-    OrderedList { items: Vec<Block> },
+    List { ordered: bool, items: Vec<Block> },
+    Null,
 }